Draw aggregate animation keys on top of folded node groups

[mihe]

Relates to godotengine/godot-proposals#1908.
Complements #113479.

This changes the drawing of folded node groups in the animation editor (as introduced by #113479) to show all keys from all tracks, but at a 30% opacity, giving you a summary overview of all the node's tracks.

Note that key links will not be drawn, nor are keys necessarily drawn the way they would be drawn in the actual track, like for example animation playback tracks and audio playback tracks. We felt that this would make things look too noisy, so keys are simply drawn as their respective track type icons.

Also note that the keys cannot be selected nor edited, they are simply drawn on top of the collapsed group.

Recording_20260311_130903.webm

[akien-mga]

Thanks!