Gizmo for RigidBody center of mass

[RBerezkin]

In Godot 4, it finally became possible to change the position of the center of mass for a rigid body. It would be nice to make this position visible in the editor window so that the user can see where it is relative to the body. Now I have to create a helper node, move it and then copy the coordinates.

[Calinou]

This is already being tracked in #2579.