road_editor.as: improved road gizmo, added mouse drag.

This is a messy commit as I reorganized the whole file into folding-regions to be able to work in script_editor.as.

New features:
* Road points now can be dragged using mouse (previously just moved with M hotkey).
* Road point gizmos now always indicate hover (previously just if unselected).
* Hover indication was changed - instead of changing color/size/thickness of the regular elements, it now draws extra elements around it.
* UI redesigned to be displayed at bottom center of screen, wide and not very tall (as opposed to previous narrow panel fixed to left side of screen) - much better for ultrawide monitors such as mine :)
* UI layout changed to accomodate new placement - multiple CollapsingHeader panels became their own tabs (mesh regen, AI waypoints).