Split Insert triplet button, and add mode menus

The "insert before" and "insert after" buttons will drop a menu to
select the new triplet mode, allowing triplets with different modes to
be inserted with one less mouse click.

The "insert copy" button still does the old behaviour of inserting a
copy of the currently selected triplet.

x26commands.cpp

@@ -72,8 +72,8 @@ void InsertTripletCommand::redo()
 
 	if (m_firstDo)
 		m_firstDo = false;
-	else
-		m_teletextDocument->emit tripletCommandHighlight(m_row+1);
+
+	m_teletextDocument->emit tripletCommandHighlight(m_row);
 }
 
 void InsertTripletCommand::undo()