Element paths

Adds path-finding support for the editor and engine. 
* pathfinding is performed in the engine. A* is used, generating new nodes on the fly via line intersection.
* test functionality using PathTester (in desktop editor tests)
* still requires polygon editing to be used from the editor; see ShapeEditor (as above) for a candidate
* perspective is set by supplying a "perspective" square
* path-finding does not yet support polygons with holes. This is a limitation of "path-is-gdx-polygon", and fixable