Update background image.

Update it accord to the design team mockup.

https://bugzilla.gnome.org/show_bug.cgi?id=664983
Update game information grid

Replace Light/Dark labels with icons. Add icon for active
player turn instead of bolding text.

Update tiles displayed in the game information grid.

Update tiles design to match the ones used by game.
Move tiles out of the theme directory so they don't get displayed
on the themes list.

https://bugzilla.gnome.org/show_bug.cgi?id=664983
Use set_accels_for_action()

https://bugzilla.gnome.org/show_bug.cgi?id=736631
Return value that didn't make sense.

https://bugzilla.gnome.org/show_bug.cgi?id=735615
Little thing in computer-player.vala.

https://bugzilla.gnome.org/show_bug.cgi?id=735615