UI: Ability to highlight vanilla priority routes when using tools

[originalfoo]

This would be useful for #7 in particular, but also generally useful.

When applying customisations with TMPE, the presence of 'priority routes' (as in the vanilla game feature, not the priority/yield signs) are invisible to the user.

It would be useful in many cases to have some visualisation of priority routes when making customisations.

As a suggestion, priority route roads could be given a yellow outline around the outer edge of the road prefab. This would keep the rest of the prefab clear for other visualisations (speed limits, etc) whilst letting the user clearly see where their priority routes are.

[originalfoo]

Rough idea of visualisation... Image below is lane highlighting, but imagine the road itself getting a border like that (not same as segment borders in Move It! mod; just a fat version of the lane highlight thing).

Image was from T++ mod: T++ source: https://github.com/joaofarias/csl-traffic

Would merit mod option (overlays tab) to choose whether it's active or not.

[originalfoo]

I consider this closed by #631.