Actual collisionchecks

With Bounce and Block actions for now.

Lines have no width.
Line -> Rectangle with a function instead of a cast and actually
returning the desired result now.
Circle shape does not do anything else than having a point and a radius,
so I changed drawing of circles and will implement circles back in once
I'm ready for more collision madness.
Way of obtaining lines from a rectangle, for inner collision checking
(like with the view).
Point.Zero and Rectangle.Zero static readonly values for comparison and
efficient access.