These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

release v0.20.13

Signed-off-by: FlashSystems <developer@flashsystems.de>

If `SimpleDatabase::Visit` is called on a database that contains a mounted directry the URIs of the elements passed to the callbacks are not prefixed by the mountpoint path. This leads to lsinfo and add not working because they use the wrong URI. This pull request is using the `WalkMount` helper function to create prefixed versions of `VisitDirectory`, `VisitSong` and `VisitPlaylist` to add the correct prefix to the parameters of the callback functions.

Closes #172

This alleviates a problem in which 'volume +1' cannot be undo by
'volume -1' when using alsa hw mixer.

Closes #104