Remove "gnarly" flipping logic.

Interface builder doesn't know to flip frames of views whose parents
have isFlipped == YES, so we do it when we're loaded from a nib.

Also compute center from frame so that it stays in sync during this
flipping and other possible view modifications.

Of course, the other option is to give in to the bottom-left coordinates
and remove the isFlipped override. Then you just need to flip your
gravity vector and everything works fine.