The ground needs to be a PhysicsBody for the collision to work.
Also added colliders to the boxes to further test collision.
The collision seems to have caused some issues with the Meldewesen
pathfinding - I think I know what's wrong, will fix soon.
The NodeGroupNotifier notifies nodes within a specific group of the
existence of a specific Node. With this new node, the Navigation does
not need to be specifically assigned to the
PathNavigatorForKinematicBody, making it easier to use in different
scenarios.
The PathNavigatorForKinematicBody node can take a KinematicBody as a
child and move it along its path. It doesn't strictly follow the path
though, but also avoids obstacles according to the Navigation (with its
Navmesh).
How the Navigation node is fetched will be changed using Groups later,
the current structure is temporary.
A PathTestWorld has been added for testing this behavior, as well as a
basic Meldewesen.
