xdg-email Fix CVE-2020-27748 (don't handle attach= in URLs)
The main objective of this MR is to fix CVE-2020-27748, it also improves handling attachments and documentation for xdg-email.
It conflicts with !58 which would completely disable the attachment support.
It does not f i x issue #205 (that is a separate MR)
When merged it will be possible to clean up a lot of code in xdg-email