This fixes a race condition in playbin2's gapless mode, where the
EOS of other streams might arrive in the sinks before the last stream
ends and the switch to the new track happens. The EOS sinks won't
accept any new data then and playback stops.
To prevent this, delay all EOS events until all streams are EOS
and advance the sinks of the EOS streams by filler newsegment
events if necessary.
Fixes bug #625118.
Before gapless playback failed when switching between audio-only,
video-only and audio-video files, when choosing different clocks
and when the different streams had different durations.
This is now handled by a helper element, which keeps track of the
running times of all streams and synchronizes them.
Fixes bug #602437.
