appdata: do not use generic name "Hitori"

In a software center like GNOME Software, we should display the more
specific name "GNOME Hitori" rather than the generic "Hitori" that we
show once we're installed.

https://bugzilla.gnome.org/show_bug.cgi?id=731308
appdata: add to GNOME project group

Some software centers, like newer versions of GNOME Software, will use
this to associate Hitori with other GNOME apps.

https://bugzilla.gnome.org/show_bug.cgi?id=731308
Use descriptive labels on board size change dialog

https://bugzilla.gnome.org/show_bug.cgi?id=731337
On game over, prompt the user to play again

This allows us to simplify the UI by obviating the need to move the new
game action from the app menu into the window. It's also simply more
useful than an OK button.

The user can instead choose to quit the app from this dialog. What a
timesaver!

https://bugzilla.gnome.org/show_bug.cgi?id=731314
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