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.h

@@ -0,0 +1,24 @@
+// SPDX-FileCopyrightText: 2019 Black Hat <bhat@encom.eu.org>
+// SPDX-License-Identifier: GPL-3.0-only
+
+#pragma once
+
+// Modified from mujx/nheko's TrayIcon.
+
+#include <QSystemTrayIcon>
+
+/**
+ * @class TrayIcon
+ *
+ * A class inheriting from QSystemTrayIcon to handle setting the system tray icon.
+ *
+ * Works for Windows, Linux and MacOS.
+ *
+ * @sa QSystemTrayIcon
+ */
+class TrayIcon : public QSystemTrayIcon
+{
+    Q_OBJECT
+public:
+    TrayIcon(QObject *parent = nullptr);
+};