Extending CustomActionsCapability etc.

[Alejandro Wainzinger]

CustomActionsCapability-based capabilities currently only activate on
right-clicking on a single track, unlike with e.g. EditCapability
which handles multiple tracks at once.  This is a bit of a request for
anyone who wants to extend CustomActionsCapability or create a new one
based on it that works on multiple tracks.  This would be required to,
for example, delete multiple files from a collection's Meta::Track
implementing a RemoveCapability of sorts, as with IpodMeta.

Also needed is a Capability perhaps called RootCapability, which only
shows on right-clicking the root of a tree (top of a collection) to
perform certain actions such as deactivation or hiding of the
collection, e.g. ipod or daap respectively.