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.

src/command_type.h

@@ -322,6 +322,7 @@ enum Commands {
 	CMD_ADD_SHARED_VEHICLE_GROUP,     ///< add all other shared vehicles to a group which are missing
 	CMD_REMOVE_ALL_VEHICLES_GROUP,    ///< remove all vehicles from a group
 	CMD_SET_GROUP_REPLACE_PROTECTION, ///< set the autoreplace-protection for a group
+	CMD_SET_GROUP_LIVERY,             ///< set the livery for a group
 
 	CMD_MOVE_ORDER,                   ///< move an order
 	CMD_CHANGE_TIMETABLE,             ///< change the timetable for a vehicle