Only store ScriptPath if fixed_script_path false

[AlexKnauth]

Adds a new fixed_script_path field similar to the fixedScriptPath field of the LiveSplit.AutoSplittingRuntime component.

When a timer program is using an externally fixed auto splitter, such as one from LiveSplit.AutoSplitters.xml matching the Game Name, it should call load_from_path with the path of that fixed auto splitter, which will set fixed_script_path to true. Then a later call to store_settings will not store the ScriptPath, consistent with LiveSplit's behavior for this case with settings stored in the splits.

When a timer program is using a local auto splitter, it should use the ScriptPath in the run's stored settings and call load, which will set fixed_script_path to false. Then a later call to store_settings will store the ScriptPath, consistent with LiveSplit.AutoSplittingRuntime's behavior when used as a Layout component instead of in the splits.