fix controller wizard menu not updating after startup

Connect to ControllerManager::mappingApplied in addition to
devicesChanged. devicesChanged is only emitted once on startup
(per the comment in controllermanager.cpp); mappingApplied fires
after a controller is opened when a mapping is applied post-startup,
so the menu now updates correctly when controllers are enabled.

Also simplify openLearningWizard loop to range-for per review.

Author: Milkii Brewster

src/controllers/dlgprefcontrollers.cpp

@@ -294,10 +294,8 @@ void DlgPrefControllers::openLearningWizard(Controller* pController) {
     }
 
     // Find the DlgPrefController for this controller by matching the controller pointer
-    for (int i = 0; i < m_controllerPages.size(); ++i) {
-        DlgPrefController* pControllerDlg = m_controllerPages.at(i);
-        if (pControllerDlg && pControllerDlg->controller() == pController) {
-            // Pass true to suppress preferences dialog when wizard closes
+    for (auto* pControllerDlg : m_controllerPages) {
+        if (pControllerDlg->controller() == pController) {
             pControllerDlg->showLearningWizard(true);
             return;
         }

src/mixxxmainwindow.cpp

@@ -956,6 +956,13 @@ void MixxxMainWindow::connectMenuBar() {
                 this,
                 &MixxxMainWindow::slotControllersChanged,
                 Qt::UniqueConnection);
+        // mappingApplied fires after a controller is opened post-startup
+        // (devicesChanged is only emitted once, on startup)
+        connect(m_pCoreServices->getControllerManager().get(),
+                &ControllerManager::mappingApplied,
+                this,
+                &MixxxMainWindow::slotUpdateControllerLearningMenu,
+                Qt::UniqueConnection);
         connect(m_pMenuBar,
                 &WMainMenuBar::openControllerLearningWizard,
                 this,