adds inter based player and source
fixes global audio caps so they match what interaudio[sink,src] expect.
audio magic.
inter: various changes.

InterSource:
    - replaced queues2 with identities for now.

InterPlayer:
    - emit periodic signal with current position.
    - adds set_mute and set_volume methods like every other source.
    - seek() now takes a parameter between 0 and 1, when going to the
      begining we use Gst.SeekFlags.ACCURATE to avoid surprises.
    - dump pipeline at start to aid debugging.
    - still there's something weird going on with the scaler.
adds widget to control playback and queueing of nonlive sources.
adds missing ui file for RecordWidget.
some ui tweaks, got rid of debugging buttons.
InterPlayer: reset position and levels on EOS
got rid of unneeded lock.
NonliveWidget: don't listen for play/pause on the player.

    seeking sets the pipeline to paused and playing, thus messing
    with the rest of the signaling system. We can always connect
    to the player directly if needed.
forgot to emit signals.
core: adds another category for video inserts.
connect debug widgets only if present, force manual by default.
make stock buttons show associated images.
remove unused callback.
adds nonlive widget and players.
ui tweaks, make it nicer when there are no cameras.