FlappyBug: Introduce a new Flappy Bug game

This introduces a Flappy Bug game. It's pretty simple currently, but is
playable.

Userland/Games/FlappyBug/Game.h

@@ -0,0 +1,99 @@
+/*
+ * Copyright (c) 2021, Mim Hufford <mim@hotmail.co.uk>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <LibGUI/Application.h>
+#include <LibGUI/Painter.h>
+#include <LibGUI/Widget.h>
+#include <LibGfx/Bitmap.h>
+#include <LibGfx/Font.h>
+#include <LibGfx/StandardCursor.h>
+
+namespace FlappyBug {
+
+class Game final : public GUI::Widget {
+    C_OBJECT(Game);
+
+public:
+    static const int game_width = 560;
+    static const int game_height = 480;
+
+private:
+    Game();
+
+    virtual void paint_event(GUI::PaintEvent&) override;
+    virtual void keydown_event(GUI::KeyEvent&) override;
+    virtual void timer_event(Core::TimerEvent&) override;
+
+    void tick();
+    void reset();
+
+    struct Bug {
+        const float x { 50 };
+        const float radius { 10 };
+        const float starting_y { 200 };
+        float y {};
+        float velocity {};
+
+        void reset()
+        {
+            y = starting_y;
+        }
+
+        Gfx::FloatRect rect() const
+        {
+            return { x - radius, y - radius, radius * 2, radius * 2 };
+        }
+
+        void flap()
+        {
+            const float flap_strength = 15.0f;
+            velocity = -flap_strength;
+        }
+
+        void fall()
+        {
+            const float gravity = 1.0f;
+            velocity += gravity;
+        }
+
+        void apply_velocity()
+        {
+            y += velocity;
+        }
+    };
+
+    struct Obstacle {
+        const float width { 20 };
+        float x;
+        float gap_top_y { 200 };
+        float gap_height { 175 };
+
+        void reset()
+        {
+            x = game_width;
+        }
+
+        Gfx::FloatRect top_rect() const
+        {
+            return { x - width, 0, width, gap_top_y };
+        }
+
+        Gfx::FloatRect bottom_rect() const
+        {
+            return { x - width, gap_top_y + gap_height, width, game_height - gap_top_y - gap_height };
+        }
+    };
+
+    Bug m_bug;
+    Obstacle m_obstacle;
+    bool m_active;
+    float m_difficulty;
+};
+
+}