trivial: Remove an empty unused switch statement
Don't show addons that are in unavailable state

This makes sure that we only show 3rd party addons when the repos that
supply the packages are actually installed and enabled.

https://bugzilla.redhat.com/show_bug.cgi?id=1145827
Drop a screenshot switching workaround now that GtkStack is fixed

... and bump the gtk+ dep to 3.14.1 for the fix.

https://bugzilla.gnome.org/show_bug.cgi?id=735749
Fix compile with appstream-glib from git master