Move remaining code to app module

There's still some stuff that could potentially go elsewhere but I think it's enough for now.

src/app/mediamanager.cpp

@@ -0,0 +1,11 @@
+// SPDX-FileCopyrightText: 2023 Tobias Fella <tobias.fella@kde.org>
+// SPDX-License-Identifier: LGPL-2.0-or-later
+
+#include "mediamanager.h"
+
+void MediaManager::startPlayback()
+{
+    Q_EMIT playbackStarted();
+}
+
+#include "moc_mediamanager.cpp"