ges_asset_request_async is unusable on custom GMainContext

The async discoverer is created & started (via gst_discoverer_start()) on ges_init() which is most likely under main thread. It makes impossible to get asset_loaded callback on non-default context (e.g., a context corresponding to g_main_context_{push,pop}_thread_default ())

I believe the timing of gst_discoverer_start() call should be delayed until it is first requested or some other way is required.