Mark or color groups differently depending on whether they're added in scene or in instance

[Zireael07]

Related to godotengine/godot#69649

(Opening here because I don't have a mockup nor code)

Personally I have been mixing and matching groups added in scene and on instance, which means I would've ran into the issue reported myself at some point.

Please either color-code them differently (Duriel suggests graying some of them out) and/or add an icon.

[Mickeon]

I think global groups would HEAVILY addresses this issue without necessarily needing more style to each group: godotengine/godot#60965.

Doesn't mean they can't be later implemented in addition to them.

• See also Allow to add descriptions to node groups in the editor #5846.