Bump required Gtk version to 3.12

https://bugzilla.gnome.org/show_bug.cgi?id=736301
Use set_accels_for_action().

https://bugzilla.gnome.org/show_bug.cgi?id=736301
Require vala 0.24

For set_accels_for_action

https://bugzilla.gnome.org/show_bug.cgi?id=736301