🎨 Various code style / behaviour fixes

- Merged platform sources when possible
- Added URLExecutableSource class
- Moved replaced_by_schema_key to utils/decorators
- Better retryable exception handling in some managers
-  Split SteamHelper into SteamFileHelper and SteamAPIHelper
- Delegated SteamRateLimiter creation to SteamAPIManager init
- Using additional_data for appid in SteamAPIManager
- Added Windows support for Legendary
- Stylistic changed suggested by pylint

src/store/managers/online_cover_manager.py

@@ -2,12 +2,11 @@ from pathlib import Path
 
 import requests
 from gi.repository import Gio
-from requests import HTTPError
-from urllib3.exceptions import SSLError
+from requests.exceptions import HTTPError, SSLError
 
 from src.game import Game
-from src.store.managers.manager import Manager
 from src.store.managers.local_cover_manager import LocalCoverManager
+from src.store.managers.manager import Manager
 from src.utils.save_cover import resize_cover, save_cover