audio: Re-add improved version of python based scanner.

- Unlike the old python version we do not wait for the first audio handoff, we
  only progress until the PAUSED state. This ensure we don't block on empty files.
- Instead of using the signal watch and running the main loop we simply poll
  the messages from the bus directly allowing for a synchronous code flow.
- Between each file the pipeline is always returned to NULL, this is done as we
  found that gst 0.10 will slow down as the uribin does not cleanup the
  children it creates for handling each file. This issue is not present in 1.0.
- This also works around a segfault that was likely caused by a race condition
  that seems to trigger in the 0.10 version of the pbutils discoverer.
- This version of the scanner also fixes the per track slow down, and works out
  to be considerably faster than even the built in discoverer from 1.0.
- Finally this removes the WMA hack as I kan no longer find any evidence of it
  being needed.
scanner: Update to use new mopidy.audio.scan

Also adds the check less than 100ms check back to the scanner.
scanner: Add progress tracking and process sorted uris
audio/scanner: Move translator into audio.

Tries to move more of the gst bits and pieces we are leaking into audio.
audio: Make min duration in scanner configurable.
audio: Don't do duration conversion to ms in scanner
audio: Add mtime to scanner results for file: uris
scanner/mpd: Add TODOs for later and fix flake8 warning
scanner: Remove unused argument for progress helper.
Merge pull request #566 from adamcik/feature/scanner

Switch back to custom scanner