audio: Add helper for converting taglists

Goal is simply to avoid leaking gst types to the rest of mopidy. Only part we
will be leaking is the tag keys. Which we can live with.
audio: Switch to using converted taglists instead of raw data.
local: Cleanup translator tests
local: Move musicbrainz cover art helper to translator
audio: Update scanner to just return tags+duration

The caller should know what URI we are talking about. Additionally finding the
mtime is never belonged in this class and has been moved out.
audio: Make scanner return duration in milliseconds

Also ensures that we normalize unknown duration to None instead of -1.
audio: Convert audio_data_to_track to tags_to_track

The new function only uses tags as input. In other words we now need to set
length, uri and mtime ourselves. Users of scan APIs have been updated.
audio: Remove min duration code from scanner.

The local scanner now handles this itself by looking at the duration.
audio: Add tags changed event to audio.

Current version simply emits the keys of the changed tags to the audio
listener. Following change will add support for storing the actual data.
audio: Fix minor typo in a debug log message
audio: Start storing the tags we find in audio

Adds a new get_currents_tags method for fetching the full set of current tags.
There are still some untested cases for this, and I also suspect we still want
some API refinements one core starts using this.
audio: Move tags to track conversion to audio utils
local: Use MIN_DURATION_MS in log message
review: Fix typos found in PR#915 feedback
audio: Fix rST formatting in docstring
stream: Fix track conversion bug and add tests

This adds basic checks for the library provider lookup:

- Check that uri schemes are respected
- Check that blacklisting and globbing works
- Check uri successfully gets converted to a track
audio: Move utils test to tests folder
audio: Only add albums that have a name

Fixes case where we could have an empty album. We could alternatively be more
conservative and only limit to fully empty albums. But I think we only want
ones with names anyway.
docs: Update changelog with audio taglist changes etc
local: Reuse the mtime we already found in local scan