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/vehicle_gui_base.h

@@ -17,7 +17,7 @@
 #include "vehicle_base.h"
 #include "vehiclelist.h"
 #include "window_gui.h"
-#include "widgets/dropdown_type.h"
+#include "dropdown_type.h"
 
 typedef GUIList<const Vehicle*, std::nullptr_t, CargoID> GUIVehicleList;