[IMP] web: Expose char_domain dialog widget

This simple return allows submodules to be able to know when a dialog is shown and modify something in it.

Note from GED: I am aware that this is a IMP in a stable version, and I really don't like that... But it looks like it really helps many people, as shown by the PR, and the risk induced by this commit is definitely extremely low, so I will make an exception.
(PR: #15579)

addons/web/static/src/js/views/form_widgets.js

@@ -501,6 +501,8 @@ var FieldCharDomain = common.AbstractField.extend(common.ReinitializeFieldMixin,
                 }
             }
         }).open();
+        this.trigger("dialog_opened", dialog);
+        return dialog;
     },
 });