[Important] Callback on UI update functionality for Nodes

[WASasquatch]

I think we should have a new method for data to get to nodes.

I propose a callback method added for nodes.

FUNCTION_CALLBACK = 'some_function'

FUNCTION_CALLBACK is ran every time a user has entered/changed the input of the node. `FUNCTION_CALLBACK`` gets the input fields, and input data like plugged node(s).

The function defined by FUNCTION_CALLBACK a @classmethod that can be used to return overwrite input field with new default data (such as populate a list input with data processed from the info inputted by the use like a system path).

I feel like callback could also be used to send arbitrary text to a container in the node (on the bottom or something) to provide descriptions, information you want to ensure the user sees that you can only print to the console, etc, etc. Said container could be hidden unless populated with a string (optionally a image path to move to temp or something?).