macosx: Refactor handling of library view segments

This PR mainly makes the segment abstraction less leaky by allowing polymorphism to take care of the specifics of the various ways in which library segments need the library window and their view controllers to be prepared

Additionally this PR fixes a few issues in the media source views due to how some of the view components have been moved around (particularly with the navigation stack)

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