Bug#1112396: dropping nautilus-sendto

[Alessandro Astone]

So I did some research on this.

The alternative used in GNOME to send files as email attachments from 
Nautilus is the org.freedesktop.portal.Email desktop portal. 
Unfortunately this uses a non-standard extension to the `mailto:` URI 
protocol to include attachments: an extension that Thunderbird, the 
default mail client in Ubuntu Desktop, does not implement -- not for 
lack of interest but for security concerns: 
https://github.com/flatpak/xdg-desktop-portal-gtk/pull/447#issuecomment-1761589783

Furthermore, if this were to actually work for sandboxed applications 
(which currently it should not), a malicious application could 
effectively read the full host filesystem by just opening `mailto:` URIs 
on itself.

nautilus-sendto itself is a fairly trivial program: it calls a 
client-specific commandline, for every mail client known to it. That of 
course means that it requires continuous maintenance for adding support 
for new mail clients, but on the other hand there is no real standard 
interface available to implement instead.

I think we'll keep using it in Ubuntu Desktop (through a patch in nautilus)