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

Welcome to the age of the Internet.
Bump the Vala and GTK+ requirements

We'll definitely need GTK+ 3.12 for all the new header bar properties
(e.g. GtkDialog's use-header-bar).

I don't think we need to require Vala 0.24 yet, so let's stick with 0.22
for now. (I'm not even sure we necessarily need 0.22 -- probably not --
but 0.16 is almost surely too old.)
Use a headerbar on the print multiple dialog
Use the term "puzzles" on the print multiple page