Controller & Eye Tracking 2 (clean PR)

[smlpt]

This an extension to #523 (the eyetracking utilization branch) and works in conjunction with this repository. This PR is a cleaned up version of #617 and #631

The changes from #617 :

• new VR behaviors like VRGrabTheWorld and VR2HandWorldTransform
• It also introduces a MultiVRButtonStateManager to distinguish between behaviors where only a single (grab) button is pressed, and one where both grab buttons are pressed. In the latter case we don't want to trigger the single button behaviors
• cell editing tools for the VR controllers (instance selection and painting, spot creation, movement and deletion)
• controller-based cell tracking
• wrist menu for triggering callbacks for Elephant actions and for playback controls and undo/redu menu
• fixed toggleVRRendering causes camera to lose arcball logic and inputhandler to lose custom keybindings #612
  introduces a button mapping manager that automatically chooses different binding layouts for different VR headsets
  spot and 3D cursor radius adjustments via the right controller joystick

From #631 :
Support for cell annotation by clustering eye gaze data. The user "counts" the cells with their eyes, we extract the fixation points and cluster them to get candidates for viewing directions with target cells. We then extract the cell positions analogous to the depth-first cell tracking approach by sampling through the volume. This will work well together with the post-processing cell merging algorithm implemented scenerygraphics/manvr3d#26

[skalarproduktraum]

Looks good, apart from some minor things (see comments)