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/logger.h

@@ -0,0 +1,9 @@
+// SPDX-FileCopyrightText: 2023 Tobias Fella <tobias.fella@kde.org>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+/**
+ * Initlalize logging to file and enables some additional categories, which will only be logged to the file
+ */
+void initLogging();