1266: Fix missing symbols in element tree

Task-Url: https://pm.winseda.de/issues/1266

css/application-dme.css

@@ -384,7 +384,7 @@ div.content-pane {
       cursor: pointer;
       padding: 2px 20px;
       white-space: nowrap; }
-      .editor-context-menu ul li.menu-item i.fas, .editor-context-menu ul li.menu-item i.far {
+      .editor-context-menu ul li.menu-item i.fas, .editor-context-menu ul li.menu-item i.far, .editor-context-menu ul li.menu-item i.fab {
         width: 25px; }
       .editor-context-menu ul li.menu-item:hover {
         background: #035151;