Check for each body individually if it collides with the other one or
ignores it.
When a body is being ignored, the other body's mass is considered
infinite when applying impulses to avoid extra overlapping.
New property disable_mode to set different behaviors:
Remove: remove from physics simulation
MakeStatic: change body mode to static (doesn't affect area and soft body)
KeepActive: do nothing
Extra change:
Handle disable/enable node state with specific notifications, in order
to differentiate global pause from disabled nodes.