Minesweeper: Create field from Difficulty enum

This change makes it easier to generate a new field. Instead of using
hard-coded values everywhere, we now just need to keep track of
the Difficulty enum value.

Userland/Games/Minesweeper/Field.h

@@ -45,13 +45,62 @@ class Field final : public GUI::Frame {
 public:
     virtual ~Field() override;
 
+    enum class Difficulty {
+        Beginner,
+        Intermediate,
+        Expert,
+        Madwoman,
+        Custom
+    };
+
+    StringView difficulty_to_string(Difficulty difficulty) const
+    {
+        switch (difficulty) {
+        case Difficulty::Beginner:
+            return "beginner"sv;
+        case Difficulty::Intermediate:
+            return "intermediate"sv;
+        case Difficulty::Expert:
+            return "expert"sv;
+        case Difficulty::Madwoman:
+            return "madwoman"sv;
+        case Difficulty::Custom:
+            return "custom"sv;
+        default:
+            VERIFY_NOT_REACHED();
+        }
+    }
+
+    Optional<Difficulty> difficulty_from_string(StringView difficulty_string) const
+    {
+        if (difficulty_string.matches("beginner"))
+            return Difficulty::Beginner;
+
+        if (difficulty_string.equals_ignoring_case("intermediate"))
+            return Difficulty::Intermediate;
+
+        if (difficulty_string.equals_ignoring_case("expert"))
+            return Difficulty::Expert;
+
+        if (difficulty_string.equals_ignoring_case("madwoman"))
+            return Difficulty::Madwoman;
+
+        if (difficulty_string.equals_ignoring_case("custom"))
+            return Difficulty::Custom;
+
+        return {};
+    }
+
+    Difficulty difficulty() const { return m_difficulty; }
     size_t rows() const { return m_rows; }
     size_t columns() const { return m_columns; }
     size_t mine_count() const { return m_mine_count; }
     int square_size() const { return 15; }
     bool is_single_chording() const { return m_single_chording; }
 
-    void set_field_size(size_t rows, size_t columns, size_t mine_count);
+    void set_field_difficulty(Difficulty difficulty);
+    void set_field_size(Difficulty difficulty, size_t rows, size_t columns, size_t mine_count);
+
     void set_single_chording(bool new_val);
 
     void reset();
@@ -87,6 +136,7 @@ private:
     };
     void set_face(Face);
 
+    Difficulty m_difficulty { Difficulty::Beginner };
     size_t m_rows { 0 };
     size_t m_columns { 0 };
     size_t m_mine_count { 0 };