FR: Event Editor as a node graph

[z64555]

Not sure why this wasn't on the issue tracker, could've sworn this was mentioned years ago when qtFred first came around.

Anyway, this FR adds/replaces the node tree used to display events with a node graph to better graphically display logic flow and event dependencies. An example of a node graph would be Blender's Blueprints editor.

[MjnMixael]

In order to do this, the sexp_tree widget will need to be untangled into a UI/Model setup so that whatever node graph UI we come up with can work with the underlying tree data the same was as the node tree does. It's on my wish list to make this happen.