For audio/video we should flush too for fastest stream switches but this
currently isn't possible because the flushes would need to go to the sink,
which then causes state changes and causes all timing information to be
changed.
Should work out of the box in 0.11 with the flush-stop that doesn't reset
the times.
Conflicts:
gst/playback/gstplaybin2.c
gst/playback/gstplaysink.c
gst/playback/gstsubtitleoverlay.c
88d3b7aeee