central locations management

atm one unknown (from the view of other plugins) component is taking care of the locations management: the xmlmenu:locations_builder. that makes it difficult for other plugins to get a full list of all locations that are currently available. (for e.g. needed by upnp_media_server)

We should have a central place, where each plugin can ask for all locations. This does not neccessary have to be build in in core, but at least a way to access it. For e.g. there could be a locations_manager-entry with a certain api in the media_manager. then different components could take care of it and the user can choose in the configuration, which one should be used....