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 they are subcategories but I prefered methods that potential users could grasp easily.  I want all the albums for an artist, here's a method that gives you a list of them (or a list-like thing; a Search Result subclasses it, you can ignore that detail).

The search_tracks method could change it's name since it could return other things, as you say.  It could just be called 'search'.