Move all D-Bus interface descriptions into a resource
Commit dbf99330 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, take this as an opportunity to stop inlining the XML altogether and load it from a resource instead.
Fixes #537 (closed)