Changes:
-Added support for custom inertia and center of mass in 3D
-Added support for custom center of mass in 2D
-Calculated center of mass from shapes in 2D (same as in 3D)
-Fixed mass properties calculation with disabled shapes in 2D/3D
-Removed first_integration which is not used in 2D and doesn't seem to
make a lot of sense (prevents omit_force_integration to work during the
first frame)
-Support for custom inertia on different axes for RigidBody3D
Use a C++ callback instead of Callable for synchronizing physics nodes' state with physics servers.
Remove usage of PhysicsDirectBodyState in physics nodes when not
necessary.
Store PhysicsDirectBodyState for bodies individually instead of a
singleton to avoid issues when accessing direct body state for multiple
bodies.
PhysicsDirectBodyState is initialized only when needed, so it doesn't
have to be created when using the physics server directly.
Move PhysicsDirectBodyState2D and PhysicsDirectBodyState3D to separate
cpp files.
