gstplayer/audio player/video player: various compatibility fixes

gstplayer:
- allow registering callbacks for async seeks/state changes
- implement gapless looping using playbin about-to-finish or
  segments
- listen to GST_MESSAGE_EOS instead of appsink eos signal,
  since appsink is only used as a video sink for video playback.
- query duration in GST_MESSAGE_DURATION_CHANGED message

audioplayers:
- actually send prepared, completed, seek completed and
  error events
- handle incorrect file source uris by audioplayers
- fix incorrect non-null terminated string printing

gstreamer_video_player:
- enable gapless looping by default
- allow overriding it in set_looping call

Author: ardera