Enhance folding in Music Library

[Laster2800]

Hi.

1. add to right-click menu:
  a) fold all
  b) unfold all
  c) expand to fold level...
2. add a setting about "default fold level"

Now the operation is too cumbersome. Everytime when switching to "no grouping" from other grouping template, you have to unfold again and agian.