Dynamic signage

[originalfoo]

This could be achieved as an extension to proposals in #49

There are already numerous mods in workshop for overhead LED signage, and with such assets (and the planned ability to change props based on road customisations) we could feasibly create 'working' dynamic overhead signs on roads.

A special tool would allow user to place the gantry, and unlit LED signs would appear above the applicable lanes or, in the case of a LED noticeboard, just the one prop angled towards the road.

Signs could update to cover congestion #76 (eg. warning to reduce speed or messaging about delays), road blockages (like 'blocked' flag when building is on fire), handle car crashes (#61), etc. They could also change based on weather or disasters, vehicle restrictions, etc.

Would be perfect for #122 tidal lanes and a whole bunch of other stuff.