Introducing the CollectionCapability.
Allows Collections to perform actions on multiple items, and after revision of CollectionTreeView, will streamline how actions are created.

Collections can now have Capabilities.

IpodCollection is the first test driver for this, with ability to delete multiple files at once.
No confirmation popup added yet, to keep translators happy.
No regressions possible, since existing other Collections will return 0 when asked for CollectionCapability.

Big commit, may require clean rebuild.