Added a signal_NodeRightClicked that is emitted when the user right click on a node

[glm-ypinczon]

The signal_NodeMoved is now only emitted on left click mouse release (was also emitted on right click)

[glm-ypinczon]

Sorry for so many pull-requests, I should have done this one at the same time than the double-click signal, but I didn't realized at this time that I would need it...