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)

Maybe pywin32 can give us access to something in windows API that will do the job, ​this is not too far off (it's only reading though).

Found ​Icon.Save in .NET

​this example writes it out straight to file.
Newer versions of windows support icons as PNGs, we should at least do that when saving .ICO fails.