Clarify contact callback handling

There are currently two types of contact reporting, onBodyContact between rigid bodies and onBodyEntered/onBodyExited for trigger bodies.

The onBodyContact callback gets triggered when bodies collide and both bodies have reportContacts set. Instead we should have two properties; sendContactReports and receiveContactReports so an object will send contact reports to other bodies and/or receive contacts from other bodies.

The trigger body is a bit different since it does not collide and thus have no contact data. Here a rigid body just needs a flag for enabling/disabling the onBodyEntered/onBodyExited callback like enableTriggerReports or something.