the newer GTK builds for win32 do not support ICO format - workaround it

[Antoine Martin]

Added a test for it so we dont flood the logs: ​changeset

GError: This build of gdk-pixbuf does not support saving the image format: ico

But we should either save it in png format (and hope the user's version of windows supports that or at least fails gracefully) or find a library that will convert it to ico (not found so far - only reading is supported, see PIL, etc)