Add error reporting to all screenshot backends, but only really use it
in the gnome-shell backend: check all DBus calls for errors instead of
crashing.
Also fix detection if gnome-shell is running: just creating the
GDBusProxy always succeeds, so call org.freedesktop.DBus.Peer.Ping
on the newly created proxy.
Some more abstraction in the screenshot plug-in, now allows for
runtime backend selection. Add stubs for the gnome-shell backend,
registers itself as unavailable because it's still unimplemented.
Jehan