Removed publicly exposed internal functions

I refactored the functions Window.InternalGetMousePosition() and Window.InternalSetMousePosition() so they are no longer publicly visible. The fix was simple, provide an internal function so the Mouse class [same assembly] can access the relative position of the window. Then change the original function [that gets overridden in derived classes] to protected so only derived classes can access it.

The only thing I was unsure about was the documentation. I left the documentation available for the protected method since that is the only method that may be exposed to the user. The other internal function [that only the Mouse class utilizes] I did not provide any documentation since the user will never see it. If you would like I can add some documentation to the other function if you think it is necessary.