Use C++17 fallthrough

src/core/mainwindow.cpp

@@ -963,7 +963,7 @@ MainWindow::MainWindow(Application *app, std::shared_ptr<SystemTrayIcon> tray_ic
       if (tray_icon_->IsSystemTrayAvailable() && tray_icon_->isVisible()) {
         break;
       }
-      // fallthrough
+      [[fallthrough]];
     case BehaviourSettingsPage::Startup_Remember:
     default: {
 

src/core/player.cpp

@@ -624,7 +624,7 @@ void Player::EngineStateChanged(const Engine::State state) {
       break;
     case Engine::Error:
       emit Error();
-      // fallthrough
+      [[fallthrough]];
     case Engine::Empty:
     case Engine::Idle:
       pause_time_ = QDateTime();

src/core/songloader.cpp

@@ -646,7 +646,7 @@ void SongLoader::EndOfStreamReached() {
       if (state_ == Finished) break;
     // It looks like a playlist, so parse it
 
-    // fallthrough
+    [[fallthrough]];
     case WaitingForData:
       // It's a playlist and we've got all the data - finish and parse it
       StopTypefindAsync(true);