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/qml/OpenFileDialog.qml

@@ -0,0 +1,14 @@
+// SPDX-FileCopyrightText: 2023 Tobias Fella <tobias.fella@kde.org>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+import QtQuick
+import QtQuick.Dialogs
+
+FileDialog {
+    id: root
+
+    signal chosen(string path)
+
+    title: i18n("Select a File")
+    onAccepted: root.chosen(selectedFile)
+}