Fix issue #3 (don't modify the user-provided dataProvider's filterFunction)

Note that the dataProvider is now the more general IList rather than
ListCollectionView.

I removed some unused imports to satisfy IDEA.

Both Flex 3 and 4 versions have had the exact same changes, but only
the Flex 4 version has been tested.
Merge pull request #4 from Dretch/dont-overwrite-filterfunction

Fix issue #3 (don't modify the user-provided dataProvider's filterFunction)