Minesweeper: Implement some feature requests.

Someone was playing this game and suggested a number of improvements so here
we go trying to address them:

- Add "chording" support, where you can click a numbered square using both
  mouse buttons simultaneously to sweep all non-flagged adjacent squares.

- Mis-flagged squares are now revealed as such on game over, with a special
  "bad flag" icon.

- The game timer now shows tenths of seconds. It also doesn't start until
  you click the first square.

- Add the three difficulty modes from the classic Windows version.

Games/Minesweeper/Field.h

@@ -2,12 +2,21 @@
 
 #include <LibGUI/GFrame.h>
 #include <LibCore/CTimer.h>
+#include <AK/Noncopyable.h>
 
-class SquareButton;
+class Field;
 class GButton;
 class GLabel;
+class SquareButton;
+class SquareLabel;
 
-struct Square {
+class Square {
+    AK_MAKE_NONCOPYABLE(Square)
+public:
+    Square() { }
+    ~Square();
+
+    Field* field { nullptr };
     bool is_swept { false };
     bool has_mine { false };
     bool has_flag { false };
@@ -15,10 +24,14 @@ struct Square {
     int column { 0 };
     int number { 0 };
     SquareButton* button { nullptr };
-    GLabel* label { nullptr };
+    SquareLabel* label { nullptr };
+
+    template<typename Callback> void for_each_neighbor(Callback);
 };
 
 class Field final : public GFrame {
+    friend class Square;
+    friend class SquareLabel;
 public:
     Field(GLabel& flag_label, GLabel& time_label, GButton& face_button, GWidget* parent);
     virtual ~Field() override;
@@ -28,19 +41,25 @@ public:
     int mine_count() const { return m_mine_count; }
     int square_size() const { return 15; }
 
+    void set_field_size(int rows, int columns, int mine_count);
+
     void reset();
 
+    Function<void()> on_size_changed;
+
 private:
     virtual void paint_event(GPaintEvent&) override;
 
     void on_square_clicked(Square&);
     void on_square_right_clicked(Square&);
+    void on_square_chorded(Square&);
     void game_over();
     void win();
     void reveal_mines();
+    void set_chord_preview(Square&, bool);
 
-    Square& square(int row, int column) { return m_squares[row * columns() + column]; }
-    const Square& square(int row, int column) const { return m_squares[row * columns() + column]; }
+    Square& square(int row, int column) { return *m_squares[row * columns() + column]; }
+    const Square& square(int row, int column) const { return *m_squares[row * columns() + column]; }
 
     void flood_fill(Square&);
 
@@ -53,15 +72,20 @@ private:
     int m_columns { 9 };
     int m_mine_count { 10 };
     int m_unswept_empties { 0 };
-    Vector<Square> m_squares;
+    Vector<OwnPtr<Square>> m_squares;
     RetainPtr<GraphicsBitmap> m_mine_bitmap;
     RetainPtr<GraphicsBitmap> m_flag_bitmap;
+    RetainPtr<GraphicsBitmap> m_badflag_bitmap;
+    RetainPtr<GraphicsBitmap> m_default_face_bitmap;
+    RetainPtr<GraphicsBitmap> m_good_face_bitmap;
+    RetainPtr<GraphicsBitmap> m_bad_face_bitmap;
     RetainPtr<GraphicsBitmap> m_number_bitmap[8];
     GButton& m_face_button;
     GLabel& m_flag_label;
     GLabel& m_time_label;
     CTimer m_timer;
-    int m_seconds_elapsed { 0 };
+    int m_time_elapsed { 0 };
     int m_flags_left { 0 };
     Face m_face { Face::Default };
+    bool m_chord_preview { false };
 };