startup: Sort application selection list alphabetically

Note that the rows need to be added to the ListBox after adding to the
dicts so that the sort function has access to them.

https://bugzilla.gnome.org/show_bug.cgi?id=732877
GTK+ required version bump to 3.12

gnome-tweak-tool is actually using a GtkSetting property that has
been added in Gtk+3.12 (and possibly others), it makes sense to bump
the required GTK+ version to 3.12

https://bugzilla.gnome.org/show_bug.cgi?id=727710