Demos+LibDesktop: Centralize screensaver logic

We have 3 demos with pretty similar window logic and quitting behavior
on user activity, so centralize that into `Desktop::Screensaver`.

Userland/Libraries/LibDesktop/Screensaver.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2022, Jelle Raaijmakers <jelle@gmta.nl>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/Function.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/Optional.h>
+#include <LibGUI/Widget.h>
+#include <LibGUI/Window.h>
+#include <LibGfx/Point.h>
+
+namespace Desktop {
+
+class Screensaver : public GUI::Widget {
+    C_OBJECT_ABSTRACT(Screensaver)
+public:
+    Function<void()> on_screensaver_exit;
+
+    static ErrorOr<NonnullRefPtr<GUI::Window>> create_window(StringView title, StringView icon);
+
+    virtual void keydown_event(GUI::KeyEvent&) override;
+    virtual void mousedown_event(GUI::MouseEvent& event) override;
+    virtual void mousemove_event(GUI::MouseEvent& event) override;
+
+private:
+    void trigger_exit();
+
+    Optional<Gfx::IntPoint> m_mouse_origin;
+};
+
+}