The receiver should not touch the old videos folder, it's not
the responsible class for that.

The VideoReceiver should work 'as is', without depending on the
whole application or in the settings. This patch removes all the
hard dependencies on QGroundControl, FactSystem and Settings system
so it's easier to isolate and test the code.

Revert "Handle gstreamer signals on main QGC thread to avoid deadlock; Remove video sink from pipeline if pipeline failed to start"

This reverts commit a9401de7.

Handle gstreamer signals on main QGC thread to avoid deadlock; Remove video sink from pipeline if pipeline failed to start

Makes more sense for it to be in VideoStreamer as it takes care of managing
the video streams.

Fix matroskamux stream start (workaround for https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/issues/449)