Sensor tree

[erincatto]

Sensors are now handled separately from regular collision. This allows sensors to be on any body type and detect any body type. Sensors are still shapes on bodies, but they don't stop checking for overlaps when the body is sleeping.
Sensors are processed at the end of the step so sensor events have no delay. Sensor events no longer need to be enabled on bodies. Instead the user should setup the appropriate filters to enable or disable sensor events.
Note: I did not test what happens if you disable a body with a sensor.

Replaced b2Timer with uint64_t
Bodies can now have names for debugging
Avoid deep clipping into chain shapes by fast bodies