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/windowcontroller.h

@@ -0,0 +1,71 @@
+// SPDX-FileCopyrightText: 2022 Nicolas Fella <nicolas.fella@gmx.de>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <QObject>
+#include <QQmlEngine>
+#include <QQuickItem>
+
+/**
+ * @class WindowController
+ *
+ * A singleton class to help manage the NeoChat window.
+ */
+class WindowController : public QObject
+{
+    Q_OBJECT
+    QML_ELEMENT
+    QML_SINGLETON
+
+    /**
+     * @brief Whether KWindowSystem specific features are available.
+     */
+    Q_PROPERTY(bool hasWindowSystem READ hasWindowSystem CONSTANT)
+
+public:
+    static WindowController &instance();
+    static WindowController *create(QQmlEngine *engine, QJSEngine *)
+    {
+        engine->setObjectOwnership(&instance(), QQmlEngine::CppOwnership);
+        return &instance();
+    }
+
+    /**
+     * @brief Set the window that the will be managed.
+     */
+    void setWindow(QWindow *window);
+
+    /**
+     * @brief Get the window that the will be managed.
+     */
+    QWindow *window() const;
+
+    /**
+     * @brief Show the window and raise to the top.
+     */
+    void showAndRaiseWindow(const QString &startupId);
+
+    bool hasWindowSystem() const;
+
+    /**
+     * @brief Set the background blur status of the given item.
+     */
+    Q_INVOKABLE void setBlur(QQuickItem *item, bool blur);
+
+    /**
+     * Toggles the window between hidden and visible.
+     */
+    void toggleWindow();
+
+Q_SIGNALS:
+    /**
+     * @brief Triggered if the managed window is changed.
+     */
+    void windowChanged();
+
+private:
+    WindowController() = default;
+
+    QWindow *m_window = nullptr;
+};