Clean up the mopidy package

- Install the LICENSE file.
- Remove makedepends, not necessary when all already are in depends.
- Remove provides, it should not contain the package name.
- Move backup- and install statements.
- Rename install to mopidy.install.
- Remove Type=simple from the service, it is the default.
- Stop the service on remove.
Make the mopidy package similar to the package for debian

- Move the config from /etc/xdg/mopidy to /etc/mopidy.
- Provide the same default config as the debian package, except for the
  logging, as systemd captures stdout/err.
- Add the man page.
- Add the .desktop file.
Update the mopidy package to 0.19.4
Merge pull request #1 from trygveaa/mopidy-fixes

Make the mopidy package similar to the package for debian, a bit clean up and update to 0.19.4