Control EventPropagation options rather than MouseFilter

[nathanfranke]

Original: godotengine/godot#16863 (comment)
WIP: https://github.com/nathanfranke/godot/tree/event-propagation

Describe the project you are working on:
Particularly UI focused applications.

Describe the problem or limitation you are having in your project:
Often, input events are captured without one knowing, especially using container nodes. The mouse filter workflow is generally confusing and needs a change of some sort.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Modification of the mouse filtering system with some key behavioural changes.

EVENT_PROPAGATION_NONE - Same as MOUSE_FILTER_STOP
EVENT_PROPAGATION_PARENT - Same as MOUSE_FILTER_PASS
EVENT_PROPAGATION_ALL - Similar to MOUSE_FILTER_IGNORE, but the event is still passed to this control. If a user doesn't want to process inputs on this control, that shouldn't be done here; one should use set_process_input.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
New Control Input Group:

Example using this hierarchy:

• Scene
  --- Red (Propagation All, shouldn't matter)
  ------ Green (Propagation All)
  ------ Blue (Propagation Parent)
  

Hovering over Blue should yield "Blue->Red"
If Blue Propagation is set to All, it should yield "Blue->Green->Red"

If this enhancement will not be used often, can it be worked around with a few lines of script?:
This should be used often, plus it is a modification to an existing API

Is there a reason why this should be core and not an add-on in the asset library?:
Mouse Filter is already core