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/async_manager.py

@@ -56,7 +56,7 @@ class AsyncManager(Manager):
     def _task_thread_func(self, _task, _source_object, data, _cancellable):
         """Task thread entry point"""
         game, additional_data, *_rest = data
-        self.execute_resilient_manager_logic(game, additional_data)
+        self.run(game, additional_data)
 
     def _task_callback(self, _source_object, _result, data):
         """Method run after the task is done"""