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/company_gui.h

@@ -13,11 +13,13 @@
 #define COMPANY_GUI_H
 
 #include "company_type.h"
+#include "group.h"
 #include "gfx_type.h"
 
 TextColour GetDrawStringCompanyColour(CompanyID company);
 void DrawCompanyIcon(CompanyID c, int x, int y);
 
+void ShowCompanyLiveryWindow(CompanyID company, GroupID group);
 void ShowCompanyStations(CompanyID company);
 void ShowCompanyFinances(CompanyID company);
 void ShowCompany(CompanyID company);