Improved how sprites can be made flippable

Changes include:
* Renamed flipped to flippedWidth, and changed comment to something more appropriate
* Added flippable property which actually is an behaves as flipped previously claimed to
* Renamed "Reverse" arguments to "Flippable"
* This change now allows sprites made using makeGraphic to be made flippable (previously not possible)
* flippable property now used instead of flipped > 0
* flippedWidth automatically updates when cachedGraphics set