Rework the well done dialog

The buttons should be actions, and sudoku is a long enough game that
there's not much value in letting players skip the one-click new game
screen. And the dialog should be modal.

https://bugzilla.gnome.org/show_bug.cgi?id=736208
Added play/pause button using original timer.

https://bugzilla.gnome.org/show_bug.cgi?id=732779
Disable the board when the game is paused

https://bugzilla.gnome.org/show_bug.cgi?id=732779
Draw overlay over the board when game is paused

https://bugzilla.gnome.org/show_bug.cgi?id=732779