Fix export kml in safari browsers

This is a fix for https://github.com/geoadmin/mf-geoadmin3/issues/1785

As written in [1] , this is quite a pain. I was able to fix this by using an older blob.js file.

As said in [2], in safari you can't specify a filename. The file will be named `Unknown` without any extension. This is @davidoesch 's prefered solution.

Another variant: The generated kml is opened in a new tab. The user then has to manually use the 'save as...' function of the browser to save it to disk.
Behaviour on other browsers should not be affected.

[1] https://github.com/eligrey/FileSaver.js/issues/12
[2] https://github.com/eligrey/FileSaver.js