Add ampache compatibility

Ampache can be accessed through Subsonic API (https://github.com/ampache/ampache/wiki/API#subsonic-api) but there was a small difference that we have to take care.

Maybe it could be interesting to specify that Clementine can use Ampache through the Subsonic service (for example, we could write "Subsonic/Ampache" title for the preference window of  Subsonic).