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/utils/steamgriddb.py

@@ -103,11 +103,11 @@ class SGDBHelper:
 
         image_trunk = shared.covers_dir / game.game_id
         still = image_trunk.with_suffix(".tiff")
-        uri_kwargs = image_trunk.with_suffix(".gif")
+        animated = image_trunk.with_suffix(".gif")
         prefer_sgdb = shared.schema.get_boolean("sgdb-prefer")
 
         # Do nothing if file present and not prefer SGDB
-        if not prefer_sgdb and (still.is_file() or uri_kwargs.is_file()):
+        if not prefer_sgdb and (still.is_file() or animated.is_file()):
             return
 
         # Get ID for the game