Refactored remaining Vec2 uses to use Vector2

[thomasvt]

Fixes # 46 https://github.com/codingben/box2d-netstandard/issues/46

I replaced all remaining uses of Vec2 with Vector2. Vec2 is deleted. This is a breaking change, as keeping the old overloads (in class DebugDraw) around didn't make much sense to me. Please advise.

I had issues with project references in both the main and ExamplesWindow solutions: project references to projects that aren't in the sln. I took the liberty to add those to the sln to allow me to build. But putting everything in one solution instead of two (current situation) is probably more elegant, if they need eachother anyway.