video interface: deobfuscate "qtwrap" layer

+ Removes the signal/slot relay mechanism(Note: Signals can be
  connected to signals and don't need a relay slot as long as the
  signature is the same).
+ renames startedDecoding to decodingStarted and the same for
  stoppedDecoding.

Depends on: https://review.jami.net/c/jami-daemon/+/21147

Gitlab: #500
Also jami-client-qt#536

Change-Id: I02746137efeefa24f8ac65bde073f230b58c1e9c