Allow linking of GraphNodes across different entities

[grtlr]

Is your feature request related to a problem? Please describe.

Right now the graph view is limited to per-entity layouts. There is also no way to reference nodes from different entities, nor entities by themselves.

By introducing cross-entity edges, we could also start to implement hierarchical graphs.

Describe the solution you'd like

It would be awesome, if entities could be treated as both:

• first class nodes (i.e. be the source and target of an edge)
• a (visual) group that contains child nodes that can be referenced globally

The simplest solution for this is to encode this information directly in the strings of the GraphEdge component (GraphNode could stay untouched).

The string could have the following interpretations:

• /<entity> prefix: An entity will be treated as a node (can be the source / target of an edge itself).
• /<entity> prefix + #<node_id> suffix: refers to a GraphNode
• all other strings: a node that lives in the same entity that this string was logged to (current behavior).

Describe alternatives you've considered

Alternative options would be:

• Having more complex types in the .fbs definitions

Open questions

• How to allow styling of entities (they don't have an associated GraphNode archetype)?

Examples

Entity-to-entity:

rr.log(`edges`, GraphEdges::new(&[("/torso", "/left_arm")]));

Node-to-node (across entities):

rr.log(`edges`, GraphEdges::new(&[("/group1#a", "/group2#b")]));