Implement arc-ball camera rotation. Use the 'a' key to switch between the

standard (up-direction fixed) and arc-ball rotation modes.

* TGLCamera
  Add functions RotateArcBall() and RotateArcBallRad().
  Add member 'Bool_t fWasArBalled'. Check its state in standard Rotate
  function and fix up direction if needed.

* TGLOrthoCamera, TGLPerspectiveCamera
  Use combined view matrix's third vector as the up direction.

* TGLEventHandler
  Add arc-ball state variable and key-handling.
  Add logic for calling std vs arc-ball camera rotate function.

* TGLSAViewer
  Document arc-ball key binding in help text.


git-svn-id: http://root.cern.ch/svn/root/trunk@44693 27541ba8-7e3a-0410-8455-c3a389f83636