Allow choose filepath

@Djedouas configuration file always contains absolute path. So, if you send the file, you have to have the same directory structure. It's possible, but not always easy. At the moment, there is no crash, but a message to indicate the problem, and you have to edit the yaml.

I propose a workaround. If a file path does not exist, one open a dialog with a file widget to choose the file/folder.