Allow for viewing entity relationships

[Freyja-moth]

From what I have seen currently the side bar that shows entities only displays Parent/Children relationships, it would however be nice to be able to select multiple different RelationshipTargets to organize the entity view by.

Something somewhat like this

// Parent/Child
UiRoot
- Button
- Text
Inventory
Item1
Item2

// Inventory/Item
UiRoot
Button
Text
Inventory
- Item1
- Item2

Theoretically it might be possible to show multiple at the same time, but that might break down when entities have multiple relationships.

[Freyja-moth]

It might be possible for each entity with a RelationshipTarget to have a small section under it that lists all relationships and allows you to expand each RelationshipTarget separately.

Not sure this is the best idea, but it certainly is an idea.

[jbuehler23]

I'll mark this down for Milestone 3 as it does seem specific for the Editor/Game interaction work

[alice-i-cecile]

This is effectively Milestone 1 work: it's for properly showing a read-only view of the scene, but it's also non-critical. Good to fix, but not blocking.