move control widgets to the right by default
make VideoMixerTransition aware of fore and background inputs, handle
linking internally.
core: don't link manually to the videomixer
update InputSelectorTransition to handle linking of sources.
use element message instead of GObject signals to handle input
disconnection.
core: some cleanup and better logging.
core: simplify code to add inputs.
factor out disconnection code, make it generic.
VideoMixerTransition: when switching to a background put it on the
correct layer.
file archiving: don't leave unconnected queues.

Previously we had a tee with a spare queue for each element and we linked to that
whenever we wanted to dump to a file. However the queues would overflow
with different amount of data, leading to a/v sync issues (one track is
shorter)

Now we just request a new pad from the tee and use a couple of queues
internal to the MuxedFileWriter.