Update to phono-backend-gstreamer 4.7.0

* Backing for the new MediaController subtitle API.
Bug Fixes
* Resolved non-reentrant PulseAudio envrionment setup.
* Disc playback (AudioCD/DVD/...) is now working correctly with more than
  one physical device.
* Prevent reading from deleted StreamReaders.
* Removed performance penalty caused by debug code even when debug was
  disabled.
* Installed icons now have the correct pixel size.
* VolumeFaderEffect can now internally abort fades when necessary.
* Removed various minor memory leaks.
* The logic behind the aboutToFinish signal has been revised. The signal
  will now only produce gapless transitions if GStreamer issued a callback
  more than 500 msec before the end of the current source. This prevents
  dead locks with short sources (e.g. notifications) but also prevents the
  queue mechanic from working when using short sources or sources of which
  the totalTime can not be identified (e.g. streams). It is advised that
  all applications make sure they handle the finished signal in addition
  to aboutToFinish to ensure that playback does not stop without user
  instruction.

(markd)