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
