Support of QUndoCommand

[tiregram]

Add the support of QundoCommand and QUndoStack to add ctrl+z/ctrl+y easily

Expected Behavior

I think we have to create:
Create_node_command
Delete_nodes_command
Move_nodes_command
Unlink_nodes_command
Link_nodes_command
Resize_nodes_command
and add the undo()/redo() action to scene