UI: Fix crash closing mixer dock panels

The function added to the BrowserDock class is SetWidget, not setWidget.

I realize now that using the same exact function name but with an
uppercase starting letter rather than a lowercase starting letter wasn't
smart.

UI/auth-mixer.cpp

@@ -204,7 +204,7 @@ void MixerAuth::LoadUI()
 	chat->setAllowedAreas(Qt::AllDockWidgetAreas);
 
 	QCefWidget *browser = cef->create_widget(nullptr, url, panel_cookies);
-	chat->setWidget(browser);
+	chat->SetWidget(browser);
 
 	main->addDockWidget(Qt::RightDockWidgetArea, chat.data());
 	chatMenu.reset(main->AddDockWidget(chat.data()));