🎨 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/async_manager.py

@@ -34,7 +34,7 @@ class AsyncManager(Manager):
         task.set_task_data((game, additional_data))
         task.run_in_thread(self._task_thread_func)
 
-    def _task_thread_func(self, _task, _source_object, data, cancellable):
+    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)