This also changes the code used to hide the custom center of mass
property for consistency with other places in the editor
(only this approach worked for Contact Monitor as well).
The Center of Mass property hint in RigidBody2D was also modified
for better usability.
Physics body previously stored the RID of a collision object and accessed it on the next frame, leading to a crash if the object had been deleted.
This PR checks the object still exists via the ObjectID prior to access.
