Handle cyclic nodes.

When you make a cycle of nodes (connecting two nodes to eachother into a cycle) the editor crashes:
```
Traceback (most recent call last):
File "/home/tuvok/Panda3d/FRAME/NodeEditor/main.py", line 52, in
base.run()
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/showbase/ShowBase.py", line 3328, in run
self.taskMgr.run()
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/task/Task.py", line 553, in run
self.step()
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/task/Task.py", line 504, in step
self.mgr.poll()
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/showbase/EventManager.py", line 49, in eventLoopTask
self.doEvents()
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/showbase/EventManager.py", line 43, in doEvents
processFunc(dequeueFunc())
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/showbase/EventManager.py", line 99, in processEvent
messenger.send(eventName, paramList)
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/showbase/Messenger.py", line 337, in send
self.__dispatch(acceptorDict, event, sentArgs, foundWatch)
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/showbase/Messenger.py", line 422, in __dispatch
result = method (*(extraArgs + sentArgs))
File "/home/tuvok/Panda3d/FRAME/NodeEditor/Panda3DNodeEditor/NodeCore/Sockets/SocketBase.py", line 70, in endPlug
base.messenger.send("connectPlugs")
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/showbase/Messenger.py", line 337, in send
self.__dispatch(acceptorDict, event, sentArgs, foundWatch)
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/showbase/Messenger.py", line 422, in __dispatch
result = method (*(extraArgs + sentArgs))
File "/home/tuvok/Panda3d/FRAME/NodeEditor/Panda3DNodeEditor/NodeCore/NodeManager.py", line 276, in connectPlugs
self.updateConnectedNodes(outSocketNode)
File "/home/tuvok/Panda3d/FRAME/NodeEditor/Panda3DNodeEditor/NodeCore/NodeManager.py", line 326, in updateConnectedNodes
self.__updateConnectedNodes(leaveNode)
File "/home/tuvok/Panda3d/FRAME/NodeEditor/Panda3DNodeEditor/NodeCore/NodeManager.py", line 337, in __updateConnectedNodes
self.__updateConnectedNodes(connector.socketB.node)
File "/home/tuvok/Panda3d/FRAME/NodeEditor/Panda3DNodeEditor/NodeCore/NodeManager.py", line 337, in __updateConnectedNodes
self.__updateConnectedNodes(connector.socketB.node)
File "/home/tuvok/Panda3d/FRAME/NodeEditor/Panda3DNodeEditor/NodeCore/NodeManager.py", line 337, in __updateConnectedNodes
self.__updateConnectedNodes(connector.socketB.node)
[Previous line repeated 972 more times]
File "/home/tuvok/Panda3d/FRAME/NodeEditor/Panda3DNodeEditor/NodeCore/NodeManager.py", line 336, in __updateConnectedNodes
connector.socketB.node.logic()
File "/home/tuvok/Panda3d/FRAME/NodeEditor/Panda3DNodeEditor/NodeCore/Nodes/TestOutNode.py", line 38, in logic
self.inputList[0].text["text"] = str(self.inputList[0].getValue())
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/gui/DirectGuiBase.py", line 446, in __setitem__
self.configure(**{key: value})
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/gui/DirectGuiBase.py", line 442, in configure
func()
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/gui/DirectFrame.py", line 99, in setText
self[component + '_text'] = text
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/gui/DirectGuiBase.py", line 446, in __setitem__
self.configure(**{key: value})
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/gui/DirectGuiBase.py", line 435, in configure
func(**options)
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/gui/OnscreenText.py", line 553, in configure
setter(value)
File "/home/tuvok/Blender/3.1/python/lib/python3.10/site-packages/direct/gui/OnscreenText.py", line 285, in setText
if sys.version_info >= (3, 0):
RecursionError: maximum recursion depth exceeded in comparison
```
The easy way out is to disallow cyclic structures and stick to a tree structure (blender colors one connection line red).
For things like a non-linear story editor, having cyclic nodes would be very interesting.