WindowServer: Add a more generic mechanism for animations

This patch adds the WindowServer::Animation class, which represents
a simple animation driven by the compositor.

An animation has a length (in milliseconds) and two hooks:

- on_update: called whenever the animation should render something.
- on_stop: called when the animation is finished and/or stopped.

This patch also ports the window minimization animation to this new
mechanism. :^)

Userland/Services/WindowServer/Animation.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <AK/Function.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+#include <LibCore/ElapsedTimer.h>
+#include <LibGfx/Forward.h>
+
+namespace WindowServer {
+
+class Compositor;
+class Screen;
+
+class Animation : public RefCounted<Animation> {
+public:
+    static NonnullRefPtr<Animation> create() { return adopt_ref(*new Animation); }
+
+    ~Animation();
+
+    bool is_running() const { return m_running; }
+
+    void start();
+    void stop();
+
+    void set_length(int length_in_ms);
+    int length() const { return m_length; }
+
+    void update(Badge<Compositor>, Gfx::Painter&, Screen&, Gfx::DisjointRectSet& flush_rects);
+
+    Function<void(float progress, Gfx::Painter&, Screen&, Gfx::DisjointRectSet& flush_rects)> on_update;
+    Function<void()> on_stop;
+
+private:
+    Animation();
+
+    Core::ElapsedTimer m_timer;
+    int m_length { 0 };
+    bool m_running { false };
+};
+
+}