Mopidy-mpris

- Remove dependency on python-indicate and libindicate, as it is deprecated and
it no longer seems to no be necessary to send a startup notification with
libindicate.

- Make tests pass with Mopidy >= 2.0.

- Make tests pass with Mopidy >= 1.1.

- No longer allow `Quit()` to shut down the Mopidy server process. Mopidy has
no public API for Mopidy extensions to shut down the server.

- Fix crash on seek event: Update `seeked` event handler to accept the
`time_position` keyword argument. Recent versions of Mopidy passes all
arguments to event handlers as keyword arguments, not positional arguments.
(Fixes: 12)

- Fix crash on tracks longer than 35 minutes: The `mpris:length` attribute in
the `Metadata` property is now typed to a 64-bit integer.

- Update `Seek()` implementation to only pass positive numbers to Mopidy, as
Mopidy 1.1 is stricter about its input validation and no longer accepts seeks
to negative positions.

- Add a hardcoded list of MIME types to the root interface
`SupportedMimeTypes` property. This is a temporary solution to be able to
play audio through UPnP using Rygel and Mopidy-MPRIS. Long term,
mopidy/mopidy812 is the proper solution. (Fixes: 7, PR: 11)

- Add a `mpris/bus_type` config value for making Mopidy-MPRIS connect to the
D-Bus system bus instead of the session bus. (Fixes: 9, PR: 10)

- Update tests to pass with Mopidy 1.0.

- Fix: Make `OpenUri()` work even if the tracklist is empty.

- Updated extension API to match Mopidy 0.18.