Fixed: - CPView nextValidKeyView infinite loop.

Previously, certain situations could easily lead to an infinite loop in - CPView nextValidKeyView. For instance, opening a sheet view with no responder returning YES for canBecomeKeyView would freeze Cappuccino.

This was caused by the cycle detection in nextValidKeyView not being able to pick up on cycles where the nextKeyView of the original receiver did not participate. Since content views often have a next key view inside of them, and the views inside rarely have a next key view back "up" to the content view, the content view would never be seen again even if there was a cycle.

This fix detects cycles at any level by remembering all previous visited responders.