Add stacked widget to edit triplet parameters

The knowledge of how colour, character etc parameters fit within the
address and data triplets is no longer required. We now present
widgets that explicitly refer to colour, character that are more
user friendly.

README.md

@@ -17,16 +17,16 @@ Although designed on and developed for Linux, the Qt 5 libraries are cross platf
 ### Linux
 Install the QtCore, QtGui and QtWidgets libraries and build headers, along with the qmake tool. Then type `qmake && make -j3` in a terminal, you can replace -j3 with the number of processor cores used for the compile process.
 
-The above will place the qteletextmaker executable in the same directory as the source, type `./qteletextmaker` in the terminal to launch. Optionally, type `make install` afterwards to place the exectuable into /usr/local/bin.
+The above will place the qteletextmaker executable in the same directory as the source, type `./qteletextmaker` in the terminal to launch. Optionally, type `make install` afterwards to place the executable into /usr/local/bin.
 
 ## Current limitations
 At the moment QTeletextMaker is in pre-alpha status so many features are not finished yet. The most-often encountered limitations are detailed here.
 
 Spacing attributes are only accessible through an "Insert" menu. A toolbar to insert spacing attributes may be added in the future.
 
-There is no visible widget for switching between subpages, use the PageUp and PageDown keys for this.
+There is no visible widget for switching between subpages, use the `PageUp` and `PageDown` keys for this.
 
-Undo only undoes keypresses, insertion of spacing attributes and CLUT changes. Other changes can't be undone, nor will they cause the "unsaved page" dialog to appear if the editor window is closed.
+Undo only undoes keypresses, insertion of spacing attributes and CLUT changes. Other changes (most notably triplet editing) can't be undone, nor will they cause the "unsaved page" dialog to appear if the editor window is closed.
 
 Keymapping between ASCII and Teletext character sets is not yet implemented e.g. in English pressing the hash key will type a pound sign instead, and in other languages the square and curly brackets keys need to be pressed to obtain the accented characters.
 
@@ -38,13 +38,27 @@ The following X/26 enhancement triplets are not rendered by the editor, although
 - Level 3.5 font style: bold, italic and proportional spacing.
 
 ## Using the X/26 triplet editor
-At present the editor only accepts raw triplet Address, Mode and Data numbers for input although the effects of the Triplet are shown in the list as a full description. The full behaviour of these enhancement triplets can be found in 
-section 12.3 of the [Enhanced Teletext specification ETS 300 706](https://web.archive.org/web/20160326062859/https://www.phecap.nl/download/enhenced-teletext-specs.pdf) but some hints and tips are below.
+The X/26 triplet editor sorts all the triplet modes available into categories selected by the triplet *type* dropdown on the left. The categories are:
+- Set Active Position
+- Row triplet
+- Column triplet
+- Object
+- Terminator
+- PDC/reserved
 
-### Address values
-Address values 0-39 select a Column Triplet and, with the exception of reserved and PDC Modes, set the column co-ordinate of the Active Position.
+Selecting "Set Active Position" or "Terminator" will change the triplet mode immediately, other selections will activate the triplet *mode* dropdown on the right which can be used to then change the triplet mode. Most triplet modes will present varying widgets below which can be used to alter the parameters of the currently selected triplet (e.g. colour or character).
 
-Address values 40-63 select a Row Triplet which has a different set of modes. Modes 4 (Set Active Position) and 1 (Full Row Colour) will set the row co-ordinate of the Active Position to the Address value minus 40, except for Address value 40 which will set the row co-ordinate to 24.
+Between the two dropdowns are the Row and Column spinboxes that are used to place the Active Position of the selected triplet, whether or not each spinbox can be altered depends on the mode of the selected triplet. As well as the explicit "Set Active Position" triplet that can set both the row and the column, all column triplets can simultaneously set the column of the Active Position. Additionally the Full Row Colour triplet can set the row of the Active Position, with the column always set to 0.
+
+By checking "raw values" it is also possible to view and edit the raw Address, Mode and Data numbers of the triplets. When editing triplets this way, remember that address values 0-39 select a column triplet which has one set of modes and address values 40-63 select a row triplet which has a different set of modes.
+
+The full behaviour of X/26 enhancement triplets can be found in section 12.3 of the [Enhanced Teletext specification ETS 300 706](https://web.archive.org/web/20160326062859/https://www.phecap.nl/download/enhenced-teletext-specs.pdf).
+
+### Setting the Active Position
+The Active Position, whether set explicitly by a row or column triplet, can only be moved in screen address order i.e. from left to right within a row and then from top to bottom across rows. In other words:
+- The Active Position can never be moved up to a lesser numbered row.
+- The Active Position can never be moved left *within the same row* to a lesser numbered column, but it can be moved left at the same time as it is moved down to a greater numbered row.
+If this rule is not followed then triplets in earlier screen addresses will be ignored.
 
 ### Objects
 "Define ... Object" triplets need to declare that they are in the correct place in the triplet list e.g. if the Define Object triplet is at `d1 t3` in the list then the data field must show `Local: d1 t3`, otherwise the Object won't appear.