Performance and resource consumption optimization for G2D picking

* Scene graph R-tree based spatial optimization of picked
  IElement-material resolution
* Parallel processing for both resolving the elements to pick and
  picking the elements
* Added INode -> IElement map management to ElementPainter to have the
  fastest possible means available in picking for getting from scene
  graph nodes to their corresponding IElement instances.

gitlab #66

Change-Id: I524da0c2695cdda2eb4f0a1fa318f48ff8fcff24