This allows a single keypress or menu click to repeatedly export a subpage
over the same .t42 file, like "Save" does for TTI files. If further teletext
page formats are added in the future this option should remember which
format was exported.
This option is deliberately different from "Save" as .t42 files are only
exported as the current subpage on view, and some metadata stored in TTI
files but not in .t42 files could be lost.
When exporting .t42 files the document is only marked as cleanly saved if it
consisted of a single subpage. Documents of more than one subpage will still
cause a confirmation dialog to be shown if unsaved, as the other subpages
will be lost unless the document is saved again as TTI.
