[#159] Use asynchronous DBus proxy creation

Created by: hedayat

This commit establishes async set up of the api and window proxy instances. In order to prevent blocking calls the actual enableing now happens in a new deferred_enable method that is called as a collback. Some extra provisions have been added to make sure that deferred_enable is executed only once even if 'called' multiple times.

This fixes gnome-shell freezes under Wayland (at least in Fedora), as it seems that it stalls gnome-shells main loop.

Fixes: #159 (closed) #198 (closed)