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.h

@@ -0,0 +1,42 @@
+// SPDX-FileCopyrightText: 2023 Tobias Fella <tobias.fella@kde.org>
+// SPDX-License-Identifier: LGPL-2.0-or-later
+
+#pragma once
+
+#include <QObject>
+#include <QQmlEngine>
+
+/**
+ * @class MediaManager
+ *
+ * Manages media playback, like voice/audio messages, videos, etc.
+ */
+class MediaManager : public QObject
+{
+    Q_OBJECT
+    QML_ELEMENT
+    QML_SINGLETON
+
+public:
+    static MediaManager &instance()
+    {
+        static MediaManager _instance;
+        return _instance;
+    }
+    static MediaManager *create(QQmlEngine *, QJSEngine *)
+    {
+        QQmlEngine::setObjectOwnership(&instance(), QQmlEngine::CppOwnership);
+        return &instance();
+    }
+
+    /**
+     * @brief Notify other objects that media playback has started.
+     */
+    Q_INVOKABLE void startPlayback();
+
+Q_SIGNALS:
+    /**
+     * @brief Emitted when any media player starts playing. Other objects should stop / pause playback.
+     */
+    void playbackStarted();
+};