Tagging media sources, configurable library views

Currently, it is a flat relation between media sources and the overall media library.

Media from all sources are scanned, and then aggregated into a single view.

Some sources may be preferred by the user due to encoding quality, or may be more closely associated with a specific human individual among several regularly operating the equipment.

An ability to associate each source with a set of tags, which may be filtered in a particular library view, would reflect this diversity of usages.

Also, as some sources may lack accurate metadata, but still be usefully accessible through file browsing, it may be helpful to allow marking certain sources as accessible for browsing, though not scanned for metadata, as are other sources.

Examples of tags include cdda-lossless, collection-of-thomas, courses, and duplicate.