Use enum for play-as setting.

https://bugzilla.gnome.org/show_bug.cgi?id=733415
Change Preferences combos and the way settings work.

https://bugzilla.gnome.org/show_bug.cgi?id=733415
Sync in Preferences dialog the main settings.

https://bugzilla.gnome.org/show_bug.cgi?id=733415
Add --level command-line option.

https://bugzilla.gnome.org/show_bug.cgi?id=733415
Add --first, --second and --two-players command-line options.

https://bugzilla.gnome.org/show_bug.cgi?id=733415
Add jumplist.

https://bugzilla.gnome.org/show_bug.cgi?id=733415