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/main.py

@@ -45,8 +45,7 @@ from src.logging.setup import log_system_info, setup_logging
 from src.preferences import PreferencesWindow
 from src.store.managers.display_manager import DisplayManager
 from src.store.managers.file_manager import FileManager
-from src.store.managers.local_cover_manager import LocalCoverManager
-from src.store.managers.online_cover_manager import OnlineCoverManager
+from src.store.managers.cover_manager import CoverManager
 from src.store.managers.sgdb_manager import SGDBManager
 from src.store.managers.steam_api_manager import SteamAPIManager
 from src.store.store import Store
@@ -97,9 +96,8 @@ class CartridgesApplication(Adw.Application):
         self.load_games_from_disk()
 
         # Add rest of the managers for game imports
-        shared.store.add_manager(LocalCoverManager())
+        shared.store.add_manager(CoverManager())
         shared.store.add_manager(SteamAPIManager())
-        shared.store.add_manager(OnlineCoverManager())
         shared.store.add_manager(SGDBManager())
         shared.store.toggle_manager_in_pipelines(FileManager, True)