- added highlight/selection handling to viewports:
 * migrated/copied some of the 3d objects highlight & selection code
from the main three-js component to the viewport element
 * modified callbacks to fit the viewport logic & the multiple scene logic:
  selections can come from any of the active & pickable scenes
 * added workaround to even normalization for the polymer-gestures lack of offsetX/Y
 * fleshed out "pickable" attribute of the three-js-scene elements
 * other minor related changed