Provide support for mounting and unmounting neighbors

[muesli4]

Task

Add a new view that let's the user see available mounts from neighbors and shows the mounted ones. Thus, provide implementations for the following commands:

• listneighbors
• listmounts
• mount
• unmount

Steps

• Check if the MPD binding library has the required commands.
• Check how we could filter out undesired mounts. For the Raspberry Pi, for example, it is unlikely that we ever would want to mount the SD card and end users should not be bothered with those partitions. A good place to implement a filter seems to be in MPD itself.
• Check how we could select a specific directory of a neighbor. The URLs themselves work but you have to know the structure in advance. The existing commands do not seem to support listing URLs with more exotic protocols. See MusicPlayerDaemon/MPD#1270.

Optional

It would be nice to have some form of auto-mounter functionality. Somehow mark certain directories and then when they appear as possible mount points mount the specific directories and execute some action for the new mountpoints. For example, add them to the queue.