remote-display: Stop watching the D-Bus name on stop

So that we don't get called out when the plugin is stopped.

https://bugzilla.redhat.com/show_bug.cgi?id=1145144
remote-display: Stop plugin when exiting

Make sure that the plugin is stopped when it's disposed of.

https://bugzilla.redhat.com/show_bug.cgi?id=1145144
remote-display: Initialise ->cancellable

At least once, right?

https://bugzilla.redhat.com/show_bug.cgi?id=1145144