Generate settings pages automatically.

This adds metadata to settings fields that can be used to automatically
generate the settings window. For now I have replaced the Difficulty
page. Will follow up to replace the others.

game/settings/booleanoption.py

@@ -0,0 +1,22 @@
+from dataclasses import dataclass, field
+from typing import Any, Optional
+
+from .optiondescription import OptionDescription, SETTING_DESCRIPTION_KEY
+
+
+@dataclass(frozen=True)
+class BooleanOption(OptionDescription):
+    pass
+
+
+def boolean_option(
+    text: str,
+    page: str,
+    section: str,
+    detail: Optional[str] = None,
+    **kwargs: Any,
+) -> bool:
+    return field(
+        metadata={SETTING_DESCRIPTION_KEY: BooleanOption(page, section, text, detail)},
+        **kwargs,
+    )