Remove usage of inline XML

Latest versions of mozjs (used by gnome-shell) drop support for E4X, so using inline XML becomes a syntax error. The DBus interfaces therefore must be declared as strings for the extension to work with latest mozjs & gnome-shell.

Tested on gnome-shell 3.11.3 with mozjs 17.0.0.

See-also: https://git.gnome.org/browse/gnome-shell/commit/?id=b908a3d70af279caad71a7ad87a301621b08271f