Feature: Option to group vehicle lists by shared orders

This applies to all kinds of vehicle lists, as well as the "vehicle groups" window.

Feature: Group liveries, and livery window usability enhancements. (#7108)

* Change: Replace checkbox in livery selection window with Default option in drop down selection.

This reduces clutter in the UI and allows for primary/secondary colours to independently follow the default scheme if desired.

* Feature: Add vehicle group liveries.