Safe margin property on CharacterBody only, used as argument in
move_and_collide.
Removed kinematic_safe_margin in 3D physics server, not really useful
and now harmonized with 2D.
MODE_DYNAMIC instead of MODE_RIGID
MODE_DYNAMIC_LOCKED instead of MODE_CHARACTER
No more special case for sleeping behavior for MODE_DYNAMIC_LOCKED
(MODE_CHARACTER was forcing the body not to sleep, which is redundant
with can_sleep and wasn't done in Bullet).