Can't add tracks to playlist from Collection panel

[Johannes Sasongko]

Test case:
- Add artist 1 to playlist.
- Add artist 2 to playlist.
- Clear playlist.
- Add artist 1 to playlist.

Problem:
- See xlgui/panel/collection @ _find_tracks.
- This function calls load_subtree to load/cache the node subtree. If the subtree is already loaded, it returns.
- However, self.tracks is only set further in load_subtree. If the subtree is already loaded, it is never set and still contains the old value.

Possible solution:
In load_subtree, if the subtree is loaded, set self.tracks before returning. (Not entirely sure that's the right approach; the code here is rather difficult to understand.)

To do:
Document self.tracks. It's unclear what it's supposed to do, yet it's used in several places.

Credits:
Mathias Brodala first reported this bug in #exaile.

[Brian Parma (bj0)]

Here's a quick fix

[Johannes Sasongko (sjohannes)]

Thanks. Assigning this to exaile-bugday for review.