Mac: [NSApp keyWindow] is not valid in windowDidBecomeKey:

This fixes weird behavior on Mac where our first responder reverts to the window
itself, rather than the SDLTranslatorResponder, after the window has lost focus
once. This causes Escape to call cancelOperation: on the NSWindow, which by
default removes our fullscreen-ness.

When someone has turned off SDL_TEXTINPUT we should probably set another initial
responder that handles the Escape behavior, so that SDL_TEXTINPUT doesn't change
fullscreen behavior (and possibly other behavior) like it does now.

--HG--
extra : rebase_source : df8cb6520810c2174f4960c263840b0ed47a757c