Fix dialog window titlebar on macOS (#4011)

After #4010

chrome/content/scaffold/load.xhtml

@@ -32,12 +32,14 @@
 <window
 	xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
 	title="&scaffold.load.title;"
+	drawintitlebar-platforms="mac"
 	onload="Scaffold_Load.onLoad()"
 	class="zotero-dialog"
 	style="min-width: 500px;">
 <dialog buttons="cancel,accept"
 	id="scaffold-load">
 	<script src="load.js"/>
+	<script src="chrome://zotero/content/titlebar.js"/>
 	<richlistbox id="listbox" class="theme-listbox"/>
 </dialog>
 </window>