Deletion implemented for WireGroups

On pressing the 'delete' key, the RigGraphicsView picks up the in the
keypressed method and checks the type of the item. This is done with if
statements, but could be neatened to polymorphism around a clear() method.
eaten when time allows. The Wiregroup is then removed from the
self.wiregroups list on the RigGraphicsView
Deletion of SuperNodeGroups implemented

Deletion is implemented on a 'delete' keypress in the same way as
implemented for WireGroups. This time using the deleteSuperNodeGroup()
method in the RigGraphics View.

However, there is no option to set the SuperNodeGroup name, so this will
now be implemented.
Naming on Drop for SuperNodeGroups

Adjusted the processDrop() method in the RigGraphicsView to detect the
drop of a SuperNodeGroup. The self.dragItem is the pin. So the pins Group
is tested (getGroup()) to see if it is a member of a superNodeGroup (it
might be a memeber of a WireGroup). If it is a memeber of a superNodeGroup
then we can use a QInputDialog to grab and check some text to apply as the
name. The name has to be unique. If the naming is cancelled, then the
SuperNodeGroup is deleted...
Merge branch 'deleteItemsBranch'