Allow custom dialogs, better styling support.

Fix registration bug with tp-form.

tp-rich-text-box.js

@@ -127,6 +127,8 @@ class TpRichTextBox extends FormElement(LitElement) {
   }
 
   firstUpdated() {
+    super.firstUpdated();
+
     // Get initial extensions from slot
     this._processChildExtensions();