Finer-grained control over layout HUD

Allows for disabling of HUD during space change, but enabling when changing layouts.

@ianyh, not sure how you feel about this implementation.  It required the least amount of code, but I don't really know in depth about how RACObserve works.  Would this cause any concurrency issues? (i.e when someone is changing spaces or cycling layouts really fast).  Assuming this is all executing within the same thread, then I don't think there should be any problems.  It may be more robust to move the displayHUD calls to the different areas of code where layoutIndex is changing and do the proper conditional checks there.

Thoughts?