08 - Provide user interaction with a 3d scene

[@7steelrainbow]

The part outlining two versions(simple/advanced) of rigid body drag handling is very helpful. Thank you!

[@TenMinutePhysics]

The demo for soft bodies is already done. Just need to find time to do the video :-)

[@mcavazotti]

A small improvement that could be done to the simple grabber: instead of setting the object origin to p_move, you could store the offset of p_start to the object's origin and when the mouse move, you could set the object origin to p_move - offset