If the module is enabled (default), 2D physics works as it did before.
If the module is disabled and no other 2D physics server is registered
(via a module or GDExtension), then we fall back to a dummy
implementation which effectively disables 2D physics functionality (and
a warning is printed).
The dummy 2D physics server can also be selected explicitly, in which
case no warning is printed.
This harmonizes the appearance of collision shapes and paths between
the editor and running project, in both 2D and 3D.
This means that in 3D, paths are now green and shapes are now cyan
instead of light blue.
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.
