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/widgets/group_widget.h

@@ -31,6 +31,7 @@ enum GroupListWidgets {
 	WID_GL_CREATE_GROUP,             ///< Create group button.
 	WID_GL_DELETE_GROUP,             ///< Delete group button.
 	WID_GL_RENAME_GROUP,             ///< Rename group button.
+	WID_GL_LIVERY_GROUP,             ///< Group livery button.
 	WID_GL_REPLACE_PROTECTION,       ///< Replace protection button.
 	WID_GL_INFO,                     ///< Group info.
 };