Finalize Custom GStreamer pipelines

OKR Goal: Finalize current work on custom GStreamer pipelines.
The main goal is to implement QTBUG-123495 (Custom GStreamer sources)

• Make player stable - Make seeking and looping stable - requires refactoring of player.

Definition of done:

• Finalize refactoring to address existing player issues
• Integrate custom sources patch (548622: GStreamer - implement custom gstreamer bin | https://codereview.qt-project.org/c/qt/qtmultimedia/+/548622)
• Document custom sources as unsupported (technical preview?), and may require help from professional services. Highlight GStreamer QML sink as a preferred API for projects that require better control of GStreamer.