Doing certain actions while rescanning collection causes tracks to be excluded

[Adam Olsen]

If you search/refresh the collection panel while rescanning (possibly only if you're scanning the collection for the first time), it causes the following exception:

Traceback (most recent call last):
  File "/home/synic/Projects/exaile/trunk/xlgui/panel/collection.py", line 140, in on_search
    self.load_tree()
  File "/home/synic/Projects/exaile/trunk/xlgui/panel/collection.py", line 347, in load_tree
    self.load_subtree(None)
  File "/home/synic/Projects/exaile/trunk/xlgui/panel/collection.py", line 468, in load_subtree
    self.tracks = self.collection.search(search)
  File "/home/synic/Projects/exaile/trunk/xl/trackdb.py", line 365, in search
    tracksres = searcher.search(query, tracks)
  File "/home/synic/Projects/exaile/trunk/xl/trackdb.py", line 627, in search
    tracks = self.__do_search(tokens, tracks)
  File "/home/synic/Projects/exaile/trunk/xl/trackdb.py", line 726, in __do_search
    for l,tr in current_list.iteritems():
RuntimeError: dictionary changed size during iteration

This causes tracks to be excluded from the final library.