Add "Exported Node" concept

[pixel-boy]

The "Editable Children" option for instanced scenes is powerful, but I never use it because, in its current state, I find it has some problems:

• Most of the time, I only want to access a few nodes
• It loses the point of having a scene packaged in a single node
• Each time I instantiate the scene, I have to recheck the Editable Children option

Much like "export" allows you to make certain properties accessible, I would like to be able to "export" certain nodes so that they appear automatically without having to check the Editable Children box, whether to access their properties or to be able to add context-specific children to them, which would add a powerful tool for compositing in Godot.

I don't think there's currently a way to do this, and I don't know how complicated/feasible it would be. Given that EditableChildren already exists, I feel like it might be not so complex to implement.

Mockup

Currently, if I want to make my Character Sprite's texture editable once the scene is instantiated, I can:

• Have a texture export variable on "Character" (which takes care of updating the child Sprite's texture) this is the solution I use all the time, even if it adds a unnecessary step.
• Use EditableChildren, which would be the best option in my option if it didn't have the flaws mentioned above.

This is what the concept of "exported node" tries to solve.

To enable the option it might look like this:

More complex hierarchy

In the case where Sprite has a parent (for example, a Pivot node), this raises the question of whether or not it should appear when the sprite is "exported." Unless this poses real problems, I think the sprite could appear directly as a child of Character, as in the mockup above.

"exported" node + Editable Children

How would using Editable Children work with this feature? Perhaps the Editable Children option shouldn't be usable in the case where a scene has exported children, or simply display the "exported" children in gray with the chain icon, as in the mockup at the very top.