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.

media/extra_grf/openttd.nfo

@@ -14,6 +14,8 @@
 // allowing it to be used.
 //
 
+//@@WARNING DISABLE 60
+
 //
 // Number of sprites, it is wrong, but GRFcodec automagically gets it right.
 //