Player: Rework dispatch_async usage

Merged Felix Paul Kühne requested to merge fkuehne/VLCKit:3.0-addasyncmediastop into 3.0

libvlc_media_player_set_media/stop are now asynchronous.

The usage of dispatch_async is not needed any more, except for the libvlc_media_player_release call.

Indeed, this function can be blocking since it can wait for the input closing.

Superseeds !71 (closed).

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