HUD api changes

I have the minimap module working again.   Engine changes needed:

Make HUDScreenLayer public api

As a bare minimum, HUDScreenLayer needs an @API marker.

CoreHudWidget maybe does as well -- at least it seemed to me that I was supposed to subclass CoreHudWidget in order to create a Minimap HUD widget.

Maybe the entire package should be accessible.

Add find element/region methods for HUDScreenLayer

I used a nasty-workaround so that I didn't need these find methods (recorded the widget returned by addHudElement and stored it in a system variable), but my code would be much cleaner and more flexible if the find* methods were in place.   We discussed needing something like this, but we didn't talk about specifics.