Avoid using experimental methods of the Web API with limited browser support

[ndavd]

Hi. Some users reported issues with the Save Current Layout functionality (see #241).

After some investigation I found that it was due to the usage of the window.showSaveFilePicker method which is not supported by most browsers at the moment.

I understand that it was probably developed with Google Chrome in mind, but it is generally a bad practice to use experimental technology that provides no advantages to stable, well-supported alternatives.