Fix app menu stateful action states

The menu was not properly displaying the state of show possible numbers
and warn on unfillable squares.
Reword the print multiple sudokus menu item

Also, use the proper ellipses character.
Remove unimplemented code for fullscreen

Let's not do fullscreen, unless we hear otherwise from the design team.
Remove several unused functions
Add modelines to every Vala file
main: remove FSF address

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