Managers refactor (#164)

The main reason for this is the compositing of both local and online covers with the same logic.  
It was a problem raised in #146 with some covers getting stretched.

Changes:
- Renamed and simplified managers methods
- Created a generic `cover manager`
- Added more retryable errors to `steam api manager`
- Removed `local cover manager` and `online cover manager`
- Reduced dependency on `PIL`

src/store/managers/file_manager.py

@@ -31,7 +31,7 @@ class FileManager(AsyncManager):
     run_after = (SteamAPIManager,)
     signals = {"save-ready"}
 
-    def manager_logic(self, game: Game, additional_data: dict) -> None:
+    def main(self, game: Game, additional_data: dict) -> None:
         if additional_data.get("skip_save"):  # Skip saving when loading games from disk
             return