Load map icons directly from DCS.

If the user's DCS directory is not configured correctly this will
degrade by not showing an icon. Otherwise (and typically) we get nicer
looking icons for each theater, and we don't have to make these for each
new map.

game/campaignloader/campaign.py

@@ -31,7 +31,7 @@ DEFAULT_BUDGET = 2000
 @dataclass(frozen=True)
 class Campaign:
     name: str
-    icon_name: str
+    menu_thumbnail_dcs_relative_path: Path
     authors: str
     description: str
 
@@ -89,7 +89,7 @@ class Campaign:
 
         return cls(
             data["name"],
-            f"Terrain_{sanitized_theater}",
+            TheaterLoader(data["theater"].lower()).menu_thumbnail_dcs_relative_path,
             data.get("authors", "???"),
             data.get("description", ""),
             (version.major, version.minor),