🎨 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/utils/steamgriddb.py

@@ -3,7 +3,7 @@ from pathlib import Path
 
 import requests
 from gi.repository import Gio
-from requests import HTTPError
+from requests.exceptions import HTTPError
 
 from src import shared
 from src.utils.create_dialog import create_dialog