Exposed _globalScreenPosition in FlxMouse so I can send low-level mouse events

I added FlxUICursor to flixel-ui:
https://github.com/HaxeFlixel/flixel-ui/commit/cadf03c1695cdb5f3438f45bc39025eac87260dc

This is intended to let the user navigate the UI using just the keyboard. I can dispatch events to the stage, but unfortunately FlxMouse resets it's X/Y position *every frame*, and ALSO keeps it hidden in a private variable. This pull request exposes that X/Y value and marks it as dangerous to mess with if you don't know what you're doing.

Here's a new demo that showcases this new feature:
https://github.com/HaxeFlixel/flixel-demos/commit/9acaefe0c834bbeded4a53a2206657e8a771a270

If/when this pull request is accepted, the FlxUICursor branches on flixel-ui and flixel-demos can be safely merged into dev.