Make plugin install foo work like awesome

* Make invokeMethod actually work by registering GstInstallPluginsReturn
as needed.
* Introduce a bool member to track whether plugin installl is active
(resets on setSource), if the member is not false when the callback
arrives playback is started immediately.
* Update the GST registry on callback arrival.
* If the plugin installer could be launched we go to a fake Loading
state until it calls back, at which point we either go to Error or
Playing.