Fixed regressions in box selection and element color function validation

* Edge element box selection didn't work because the contained picking
  logic was not implemented properly in the edge element class.
* Element coloring brightness function validation did not set "graph"
  key for SCLContext which caused compilation to fail occasionally which
  broke the function evaluation.
* Fixed vertex selection visualization to stroke the edge of the normal
  rendered rectangular shape which is much cleaner than the old default
  selection visualization that was geometrically out of place for some
  reason.

gitlab #2

Change-Id: Ib1f5e03a1eeb6101ffe0f527c5a54ece97e1ea8e