Add a D-Bus API that allows the API user to introspect the application
state of the shell. Currently the only exposed information is list of
running applications and which one is active (i.e. has focus).
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/326
(cherry picked from commit 10c68c6b952959d105c3126fc61f22a199e8e848)
