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/optiondescription.py

@@ -0,0 +1,13 @@
+from dataclasses import dataclass
+from typing import Optional
+
+
+SETTING_DESCRIPTION_KEY = "DCS_LIBERATION_SETTING_DESCRIPTION_KEY"
+
+
+@dataclass(frozen=True)
+class OptionDescription:
+    page: str
+    section: str
+    text: str
+    detail: Optional[str]