[*] each Collection can have only one Indexer. it can even be reused.
[*] create the test files with make.
[-] the tests files are created by the Makefile now.
[*] s/not_index/do_not_index/.
[+] use a CollectionUpdater. stop it at __del__(), otherwise we 'leak' the thread and the process never finishes.
[*] return abspaths.
[*] return abspaths. [*] only stop the notifier if it hasn't been stopped before.
[+] test_new_song() and test_new_wrong_file() make sure we can see new files but of the right type.
[*] cosmetic.
[+] CollectionUpdater also notifies Song deletions.
[+] also test that removing a file is not notified.