World management library no longer uses Rectangle2D from the Java standard API

-Added Rect2D class to world package; represents a rectangle in world
space (coordinates at bottom left corner - ascending Y axis)
-Reworked Entity/World2D/WorldImpl classes and all existing demos to use
Rect2D in place of Rectangle2D.
Merge branch 'master' of https://github.com/bgroenks96/2DX-GL