Codechange: Switch DropDownList to directly use std::vector, thus making AutoDeleteSmallVector obsolete.

DropDownListItem are strongly managed using std::unique_ptr to ensure leak-free handling. Appropriate use
of move-semantics make intent a lot clearer than parameter comments and allows the compiler to generate
copy-free code for most situations.

src/vehicle_gui_base.h

@@ -47,7 +47,7 @@ struct BaseVehicleListWindow : public Window {
 	void SortVehicleList();
 	void BuildVehicleList();
 	Dimension GetActionDropdownSize(bool show_autoreplace, bool show_group);
-	DropDownList *BuildActionDropdownList(bool show_autoreplace, bool show_group);
+	DropDownList BuildActionDropdownList(bool show_autoreplace, bool show_group);
 };
 
 uint GetVehicleListHeight(VehicleType type, uint divisor = 1);