Add Static Methods: CurrentHoverControl() and CurrentFocusControl(), along with ChangeEvents.

[najak3d]

This is similar to a question I asked a few months ago. Now I'm asking for this Feature.

Rather than have each user do a Ray query per frame update to determine the visible "HoverControl()", it would be very nice if this were just built into NovaUI. (Then you might optimize it, if that helps, rather than running the full query ever frame.)

And also, it would be very convenient if we could just query for "CurrentFocusControl()" so that the rest of our system can ignore Keyboard input, until this value is Null.

Each of these should also have a static ChangeEvent, so that when either value changes, it notifies listeners.