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/group_gui.cpp

@@ -768,8 +768,7 @@ public:
 				break;
 
 			case WID_GL_MANAGE_VEHICLES_DROPDOWN: {
-				DropDownList *list = this->BuildActionDropdownList(true, Group::IsValidID(this->vli.index));
-				ShowDropDownList(this, list, 0, WID_GL_MANAGE_VEHICLES_DROPDOWN);
+				ShowDropDownList(this, this->BuildActionDropdownList(true, Group::IsValidID(this->vli.index)), 0, WID_GL_MANAGE_VEHICLES_DROPDOWN);
 				break;
 			}