Add additional semantic information on hovering graph nodes / edges

[grtlr]

It would be very helpful to highlight semantically relevant nodes (or edges) to an item that is currently hovered. The most basic examples would be to highlight adjacent nodes / edges in a secondary color.

Relevant discussion on Discord here: https://discord.com/channels/1062300748202921994/1320772574271115384/1320772574271115384

Maybe we could have a hovering mode, set via blueprints, that defines how the hovering should behave:

• Hovering highlights adjacent nodes
• Hovering highlights reachable nodes
• ...

To implement this, we would have to actually parse the resulting (combined) graph into petgraph for efficient queries. This is a little bit more involved, as petgraph does not have subgraphs, if I remember correctly. Also we would need to add the hover information to the state object, due to the immediate mode rendering. Note that in some cases, this might lead to a one-frame rendering delay.