Refactor window handling code

Currently when we want to show/raise the window in reaction to the tray icon/notification being clicked etc we do this by emitting a signal on the controller.
This is connected to in main.qml, which does some things, then calls back to controller to do more things.

This is quite convoluted. Instead introduce a new class WindowController that is responsible for all things window, in particular showing/raising and config saving

src/windowcontroller.h

@@ -0,0 +1,26 @@
+// SPDX-FileCopyrightText: 2022 Nicolas Fella <nicolas.fella@gmx.de>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <QObject>
+#include <QWindow>
+
+class WindowController : public QObject
+{
+    Q_OBJECT
+
+public:
+    static WindowController &instance();
+
+    void setWindow(QWindow *window);
+
+    void restoreGeometry();
+    void saveGeometry();
+    void showAndRaiseWindow(const QString &xdgActivationToken);
+
+private:
+    WindowController() = default;
+
+    QWindow *m_window = nullptr;
+};