Room Editor: Add ability to create and organise groups in the layer instances/assets panel

[nullbunny]

Is your feature request related to a problem?

I really love the new pop-out panel for instance layers - It makes layers with many instances so much more manageable. Would it be possible to get a layer-intern groups for Instance and Asset layers?

Describe the solution you'd like

It would be a pure organizational tool in the Room Editor to avoid introducing more Instance layers or Asset layers that aren't really needed.

For example, let's say you have an Asset layer with decorations for the foreground - We could use those groups to organize different types of decorations like flowers, rocks, etc. instead of introducing actual layers at such a granual level. Some of my decorative layers have hundreds of assets each.

Describe alternatives you've considered

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Additional context

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