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
We'll soon move the inlined XML of all interfaces we export or consume into
separate files. That's a significant increase from the interfaces for which
we install documentation, so it makes sense to use a dedicated subdir.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/537