Introduce MediaManager

Currently, this is only to pause playback of other media when something new starts playing.
In the future, it will also be used for other things, like call ringing, etc.

src/mediamanager.cpp

@@ -0,0 +1,9 @@
+// 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();
+}