Codechange: Avoid emplace_back(new()) into a unique_ptr. (#11384)

This could theoretically leave an unmanaged pointer in certain circumstances, and directly using
make_unique shows intent.

src/textfile_gui.cpp

@@ -528,8 +528,7 @@ void TextfileWindow::AfterLoadMarkdown()
 			DropDownList list;
 			for (size_t line : this->jumplist) {
 				SetDParamStr(0, this->lines[line].text);
-				DropDownListStringItem *item = new DropDownListStringItem(STR_TEXTFILE_JUMPLIST_ITEM, (int)line, false);
-				list.emplace_back(item);
+				list.push_back(std::make_unique<DropDownListStringItem>(STR_TEXTFILE_JUMPLIST_ITEM, (int)line, false));
 			}
 			ShowDropDownList(this, std::move(list), -1, widget);
 			break;