Alternative search musiclibrary

This PR is NOT meant for pull consideration. It is only made as a basis for discussion for pull request #246.
Yes, well. As far as I can tell, the '/' does not really do and exact search when searched/browsed, it browses/searches it as a subcategory. So e.g.:

A:ALBUMARTIST:Metallica performs a fuzzy search among albumartists for metallica and returns artists
A:ALBUMARTIST/Metallica looks for the content of the Metallica subcategory to albumartists, and therefore returns albums.

The point is, that these functions will then return items with a different type than the one suggested by their name, which means that I don't think we can talk about a 'search', so if this feature is added it should be a separate argument like "subcategories". I don't mind giving it a whirl (as an example), but as I mentioned elsewhere, I really only created this PR to show what I think is a better way to implement those specialized searches. You already did all the hard work with the escaping, so I think it should be your PR/branch we continue with.