Use gtk_application_inhibit()

Now that we require Gtk 3.4 it makes sense use the inhibit
functionality provided by GtkApplication rather than doing the DBus
calls ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=735093
Change GtkApplicationInhibit flags

It makes more sense to inhibit switching users rather than inhibiting
marking the session as idle.

https://bugzilla.gnome.org/show_bug.cgi?id=735093
Use g_slist_free_full() to free lists

Rather than using g_slist_foreach() followed by g_slist_free() to free
a list and it's data just call g_slist_free_full().

https://bugzilla.gnome.org/show_bug.cgi?id=735092
Release 3.11.91