Make App dbus-launchable

We should make the app dbus-launchable. Besides providing this as a general GNOME best practice this will also build the foundation to allow other apps such as the shell-extension to call future dbus methods of our app.

Relevant docs:
- https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s07.html
- http://raphael.slinckx.net/blog/documents/dbus-tutorial