Entity Path syntax highlighting

[emilk]

Whenever we show an entity path in the UI we should use syntax highlighting:

• the path separator / should use a different color, and probably have an extra half-space on each side
• any escape sequence should have a different color to indicate that the \ is not a literal backslash

[emilk]

I started working a bit on this in the emilk/syntax-highlighting branch, but there is a snag: sometimes an entity path is shown as an noninteractiver label, and sometimes as an interactive button, and that means different text colors. I'm not sure how to nicely handle that. How does the syntax highlighter know if it is highlighting for a label or a button?

One option it to use a special None color for the bulk of the path, and then have egui replace that with the "current" text color when painted (requires an egui change). Slashes and escapes would always have the same color, regardless of where they are.

If we ever implement emilk/egui#3284 then we could use a "tint" modifier or something instead of hard-coded colors, but we're not there.

[emilk]

This will be unblocked by:

• Replace a special Color32::PLACEHOLDER with widget fallback color emilk/egui#3727
• Update to egui 0.25 and winit 0.29 #4732