[Feature] Add support for partitions

[GioF71]

We need support for allowing the definition of additional partitions.
See here

[GioF71]

Progress: after more thinking, this will be solved with an external python script. The only thing that can be statically configured is the list of partitions, but those can be dynamically created after mpd has started.

[GioF71]

Made some testing with a branch in upmpdcli built to differentiate avtransport and openhome play queues using partitions. Purpose is to avoid play queue clashes when switch from av to openhome and vice versa.
It does work, but not reliably across restarts, mainly (I think) for these reasons:

• partitions are not persistent
• the fact that an output belongs to a partition is not persistent (hence the python tool I added)
• at restart, if the script if not run, upmpdcli does not find the partitions and/or does not have an output associated

so probably upmpdcli should also create the partition before trying to use them, and maybe associate a configured output. But this extends the changeset. I am not even sure that the play queue is persisted on the state file for the partition, but given that partitions are not persistent, I doubt it.

Furthermore: using partitions breaks most scrobblers AFAIK because when playing on a partition, the scrobbler (unless it explicitly supports partitions and such partition is configured) is not aware of the fact that something is playing.

So, currently, as painful as it is for me, IMO, the best solution to avoid play queue issues if one want both av and openhome, is to create two sets of apps (mpd+upmpdcli+scrobbler if needed), one for av, the other for openhome. It's quite a waste of resources and requires more configuration.

I'd be happy to understand that I am overlooking something, so I would be happy to return on this. Contributions are welcome.