Fix the start icon in RTL

https://bugzilla.gnome.org/show_bug.cgi?id=722302
Use symbolic icons

https://bugzilla.gnome.org/show_bug.cgi?id=722302
Remove relief from pause/play button

This looks a bit better and is more consistent with the other games.

https://bugzilla.gnome.org/show_bug.cgi?id=722305