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
Commit dbf993300a moved all inline D-Bus interface descriptions to template
strings so we can stop escaping line breaks.
Unfortunately that unveiled a grave bug in xgettext, which currently cannot
handle files that contain both backtick and slash characters - as a result,
translations from affected files have started to disappear as translators
run xgettext/msgmerge.
Instead of reverting the change and getting the crusty escaping back, we
will take this as an opportunity to stop inlining the XML altogether and
load it from a resource instead.
To facilitate that, add a small helper method that loads a D-Bus interface
description from a dedicated resource bundle.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/537
