ext: Add library updaters to extensionss
local: Switch to using updater from extensions
local: Remove updater from backends
backends: Add change track helper to playback provider
utils: Switch to urlparse for file-uri/path handling
utils: Convert path_to_uri to single argument
local: Convert mopidy-local extension to local: uris.

- Use local:playlist:<name>, local:track:<path> and local:search uris
- Adds LocalPlaybackProvider which translates to file uris.
- Switches to storing actual uris in playlists - so local: urls and not
  file:// or plain paths.
- Moved file:// to streaming plugin
- Cleaned up tests and imports for these changes.
local: Review fixes.
Merge branch 'develop' into feature/local-uris
docs: Update changelog with respect to local uri scheme and plugable library updaters.
Merge pull request #482 from adamcik/feature/local-uris

Switch local over to custom uris and make library updater plugable.