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/store.py

@@ -130,7 +130,7 @@ class Store:
         # Connect signals
         for manager in self.managers.values():
             for signal in manager.signals:
-                game.connect(signal, manager.execute_resilient_manager_logic)
+                game.connect(signal, manager.run)
 
         # Add the game to the store
         if not game.source in self.source_games: