Image name is not sanitized, corrupting XML

[JbIPS]

Hi,

I just tried to create a dynamic wallpaper and I constantly got a corrupted file after save ('Corrupted file' error message on re-open).

After some investigation, I noticed that the culprit is an image with a & in its name. Opening the resulting XML, I saw the & is not escaped, thus creating a corrupted XML file.

I think you need an escape mechanism (maybe just xml.sax.saxutils.escape(data, entities={})) when exporting to XML. And of course, unescape on load.

I don't know if it has some border effects but if you think it's OK, I can provide a PR.

Thank you for this editor!