Draft: Interactor style resembling common 3d editors such as Blender

This is a rewrite of: https://github.com/RubendeBruin/DAVE/blob/master/src/DAVE/visual_helpers/vtkBlenderLikeInteractionStyle.py

The idea is to create an interactor that works like blender (and many other programs).

• Selection using left button
• Right button reserved for context menu
• Navigation using middle button with
  • pan
  • zoom
  • rotation about the Z axis in a turn-table fashion
  • when zooming using mouse-wheel: pan towards or away from cursor position

And then (copied behaviour):

• selecting actors using a rubber band
• grabbing and dragging actors (need to have a look at where this is already implemented, the widget is nice)
• measuring a distance on the screen (in-plane)

The current state has a working implementation of the rotation. Everything else is just a copy-paste of the TrackballActor interactor. But I'm glad I got this far. General feedback is welcome :-)

@mwestphal