Big chunk commit on PhysicsTerrain integration:
- added PhysicsTerrain via bullet
- redesigned agent<->model linkage logic
- added mass attribute to PhysicsModel serialization, implemented loading
- integrated bullet debugDrawer
- refactored Editor::agentIdUnderMouse and modelIdUnderMouse for easier use
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Added isValid to ModelManager interface for public access.
Fixed crash if calling isValid on a newly created ModelManager
- added handling of invalid modelId when linking with an agent
- agents linked to 0 models are now deleted right away