Create a Custom Events system.

[esklarski]

For now it does nothing, but it doesn't crash either :)

I decided to clean my working tree and start anew in my attempt to create a Goto block to enable execution at arbitrary points of an orchestration.

TODO:

• create a node for this panel to instantiate
• figure out how to integrate these "events" into method call registration so they can be called from GDScript
• cleanup

Many TODO notes to resolve so this is a work in progress, but working better than my first attempt.

This replaces #820