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/display_manager.py

@@ -30,7 +30,7 @@ class DisplayManager(Manager):
     run_after = (SteamAPIManager, SGDBManager)
     signals = {"update-ready"}
 
-    def manager_logic(self, game: Game, _additional_data: dict) -> None:
+    def main(self, game: Game, _additional_data: dict) -> None:
         if game.get_parent():
             game.get_parent().get_parent().remove(game)
             if game.get_parent():