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/trayicon_sni.h

@@ -0,0 +1,30 @@
+// SPDX-FileCopyrightText: 2021 Aleix Pol Gonzalez <aleixpol@kde.org>
+// SPDX-License-Identifier: GPL-3.0-only
+
+#pragma once
+
+#include <KStatusNotifierItem>
+
+/**
+ * @class TrayIcon
+ *
+ * A class inheriting KStatusNotifierItem to provide a tray icon.
+ *
+ * @sa KStatusNotifierItem
+ */
+class TrayIcon : public KStatusNotifierItem
+{
+    Q_OBJECT
+public:
+    explicit TrayIcon(QObject *parent = nullptr);
+
+    /**
+     * @brief Show the tray icon.
+     */
+    void show();
+
+    /**
+     * @brief Hide the tray icon.
+     */
+    void hide();
+};