fix 'mousedown' in _downHandler

'mouseup' was dispatched instead of 'mousedown' in _downHandler
add right click events

Add rightClick, rightClickStage, rightClickNode, and rightClickNodes
events
Revert the deletion of examples/drag-nodes.html
Merge remote-tracking branch 'origin/master' into captors-mouse-right-click
Merge remote-tracking branch 'origin/master' into lk-sigma
Merge remote-tracking branch 'origin/captors-mouse-right-click' into lk-sigma