Integrate playlist with GstDiscoverer API

[Rafostar]

For local media, showing file name as title was good enough, but since now we have and use for web content gtuber plugin that works nicely with GstDiscoverer, it would probably be plausible to run discoverer when new video is added to the playlist. This way we can show actual video title in it instead of URI which is not very meaningful. The discoverer should probably be run on a new thread invoke, so it won't affect current video playback.