- added basis for "mirror" operation (not functional yet)
- updated selected objects handling via the new selectionsAdded/
selectionsRemoved events from the polymer-threejs:
 * attempting to implement group transforms (multi select) : not there yet !
- added various ui elements for export format selection
- various minor adjustments , experiments etc
- added support for undo -redo operation events
- added multiple format support for export, cleaned up handling of exports