Fix 2 startup races when things happen too quickly, and 1
at shutdown by holding a ref to the pads in use until the
loop functions exit.
Handle errors activating file parts and publish them on
the bus.
https://bugzilla.gnome.org/show_bug.cgi?id=750747
Implement 2 new elements - splitmuxsink and splitmuxsrc.
splitmuxsink is a bin which wraps a muxer and takes 1 video stream,
plus audio/subtitle streams, and starts a new file
whenever necessary to avoid overrunning a threshold of either bytes
or time. New files are started at a keyframe, and corresponding audio
and subtitle streams are split at packet boundaries to match
video GOP timestamps.
splitmuxsrc is a corresponding source element which handles
the splitmux:// URL and plays back all component files,
reconstructing the original elementary streams as it goes.
