Implement macOS cold start fix in GUI_App to store URL for post-initi…

[Arlieeee]

…alization handling, ensuring consistent behavior with Windows.
This pull request addresses an issue with opening URLs on macOS during a cold start in the GUI_App class. The update ensures that URLs opened before the application is fully initialized are handled consistently with Windows, preventing the model from being cleared unintentionally.

macOS cold start handling improvements:

• Modified GUI_App::MacOpenURL in src/slic3r/GUI/GUI_App.cpp to store the URL in init_params->input_files if the application is not yet post-initialized, ensuring the model is not cleared and the URL is properly processed after initialization. This aligns macOS behavior with Windows and prevents loss of the model to be loaded from the URL.