Custom Node Creation Menu Items

[dannymate]

Added attribute for setting methods as custom node creation menu items. Same as the NodeMenuItem.

Switched return type in BaseGraphView & NodeProvider to string, Type, Func<Type, Vector2, BaseNode> to match my new methods for returning the new attribute methods. This way we enforce all attributes to the correct Node creation format and can use the BaseNode.CreateNode method.

I haven't implemented graph specificity yet.

I've also added an example.

I imagine this will be a breaking change for GraphViews.