Improve Add Node Workflow in Graph Editor (#2383)

Continuing on from @beersandrew 's early work on adding the ability to add a node by drawing a link from a pin.

For reference, this is @beersandrew 's original PR: #2157

What I have added/modified so far:
* Improvement on @beersandrew 's filtering logic for simpler if statements and also for categorized filtered candidate nodes in the tree menu
* Filter out nodes having no inputs of matching type rather than filtering by node types only as per @beersandrew 's original plan
* Added the logic to automatically link the newly created node back to the original pin, from which the link was drawn from

Also done the reverse direction too.

* Distinguish if the currently drawn link is forward or backward
* Candidate nodes are filtered based on output types if backward
* Connect the original input pin back to the output pin of the newly created node

Author: kiyoyukinakagaki-fn