Codechange: Move dropdown and slider out of widgets directory. (#12403)

Also shuffle headers to place widget includes near end.

This leaves the widgets directory solely for defining Widget IDs.

src/ground_vehicle.hpp

@@ -14,6 +14,7 @@
 #include "vehicle_gui.h"
 #include "landscape.h"
 #include "window_func.h"
+
 #include "widgets/vehicle_widget.h"
 
 /** What is the status of our acceleration? */