Add support for saving files on desktop.

Added FilePickerCross.save() method its purpose is to open a file create system dialog.

This PR only covers desktop portion of the solution.

I have looked into doing a mobile version, but have decided to not proceed for now since it seems that it may not be universally available, my thinking is that either this whole PR is rejected. A partial implementation is done that explicitly only supports desktop clients or a compromise solution is suggested for mobile implementation.