Replace deprecated properties to fix warning

https://bugzilla.gnome.org/show_bug.cgi?id=737045
Use set_accels_for_action ()

https://bugzilla.gnome.org/show_bug.cgi?id=737046
Require vala 0.24

Maybe needed for set_accels_for_action. I don't care to look up what
0.23 release that was added in.

https://bugzilla.gnome.org/show_bug.cgi?id=737046