macosx: Extend use of inheritance and polymorphism with library data types, eliminate code duplication

With the introduction of VLCMediaLibraryItemProtocol we cut down on a lot of duplication when handling different media library item types, however by only having a protocol we still need to duplicate several things within the class implementation themselves. By introducing new abstract base classes with shared implementations for certain properties and methods, we can cut down on duplication even further.

With VLCLocallyManipulatableItemProtocol we can also extend polymorphism to things like revealing in finder to VLCInputItem as well as VLCMediaLibraryMediaItem

Signed-off-by: Claudio Cambra developer@claudiocambra.com