Make it possible to add actions of a special PopupDropperAction subclass to a list of global actions that will be applied to all items of a specific meta type in all collections.

This is useful, for instance, for services that can offer special functionality related to some items, like last.fm that can provide a simmilar artist stream for any artist.

The last.fm service now uses this system which means that the action is available everywhere (even in other services) and that the action is no longer hardcoded anywhere in the core of Amarok.