Reduce opacity of nodes with no shared transition when selecting an AnimationNodeStateMachine node

[vaner-org]

• Closes 10976
• Also adds this functionality when selecting transition lines

This is intended to help visibility when working with complex AnimationNodeStateMachine cases.
This PR is a companion to 98401, which makes the selection of overlapping transition lines unambiguous.

highlight-selected-neighbours.mp4

As this is my second PR, criticism and feedback would be very much appreciated. I was a bit clueless on header organization and function placement so please let me know what I did wrong.

[vaner-org]

In my last commit, I made it so that when a pair of transitions exist, selecting one also highlights the other.

[fire]

I'll bring this to the Godot Engine animation meeting later this week to review.

[SaracenOne]

I think this is a reasonable improvement to the usability of the state machine editor. I would suggest squashing the commits into a single commit though before we merge it, as this is the preferred way of merging PRs.

[vaner-org]

All done, ready for you.

[akien-mga]

Thanks!