Initial mediaplayer implementation

Initial mediaplayer implementation

Notable changes:
- Added dart_mpd package as a dependency.
- Added MPD client class and associated provider.
- Added MPD client configuration to the configuration file for
  potential usecases where MPD may not be available locally.
- Added playlist, play state, art, etc. providers for use in the
  mediaplayer front end UI.
- Reworked MediaPlayer classes to wire up MPD client backend.
- Removed volume slider from the bottom of the media pages to
  make more room for playlist / preset tables, as only being
  able to show 3 entries in each was not usable in practice.
- Reworked media player mocked up position indicator into an
  actual slider control, and reworked the radio slider styling
  to match for better UI consistency.
- Reworked media player and radio playlist / preset tables to
  attempt to have them layout at the same location and add an
  always visible scroll bar.  The scroll bars currently have a
  layout issue with respect to scroll track size that does not
  seem to have an obvious fix.  They are usable for now, and
  further investigation will be done when time permits.
- Wired up play/pause button on the side volume control via a
  new set of play state and controller providers.

Bug-AGL: SPEC-5028, SPEC-5029

Change-Id: I87efecc58b4e185443942eb32ff8148ebcd675c3
Signed-off-by: Scott Murray <scott.murray@konsulko.com>