
Navigation:
 left button:           rotate around the target
 middle button:         zoom
 left button + ctrl     quake rotate (rotate around camera position)
 middle button + ctrl   walk (progress along camera's z direction)
 left button:           pan (translate in the XY camera's plane)

R : move the camera to initial position
A : start/stop animation
C : clear the animation
G : add a key frame

