3pt update

Created by: zeffii

the default UI for this node doesn't make a whole lot of sense, we can use the quicklink mechanism to inject a vector node into the 3pt input socket. It might be interesting and beneficial for other nodes to also offer a way for nodes to override the exact kind of quicklink behaviour.

The initial intent of the quicklink handler was to allow specific behaviour that makes sense to individual nodes. A quicklink for the Rotation node isn't the same behaviour you might want for the 3pt_node, proof is the issue posted by @Durman .

This pr updates the 3pt node and now will offer a custom function (defined in the node) to set up your own quicklink behaviour.

• new general socket property called "custom_quicklink_function_name", it will (obviously?) store the name of the function (located on the node) that will be used to draw the socket ui row.
• when the above property is not "", then the function will be called in a try/except by the quicklink pathway in node_tree.py -

     def draw_quick_link(self, context, layout, node):
         ...

• if there are errors in the UI draw call, then the default quicklink for that sockettype will be drawn.

no ETA.