Fixed: next/previous key view could be from another window

Previously, Cappuccino would allow a next/previous key view to belong to a different window, which could result in an infinite loop.

Attempting to prevent this when setting the next/previous key view broke the responder chain, because sometimes the responder chain is being constructed before the view has a window.

This commit defers the check for next/previous views belonging to the same window to when they are requested.