Updated HeaderBar to show puzzle difficulty as subtitle

Also made the difficulty strings translatable

https://bugzilla.gnome.org/show_bug.cgi?id=710624
Add keywords to desktop file
Update TODO
Removed 'Hint' and 'Help' buttons and related code

https://bugzilla.gnome.org/show_bug.cgi?id=710624
Fix window icon
popover non-modal
SudokuCellView: clean up number picker code

In particular, do not repurpose and destroy widgets. Just keep as many
as needed, and hide them when not in uses.