Add compatibility with Thunderbird to send emails with attachment

[mhalano]

Today, Thunderbird can't send an email with an attachment using libportal. GNOME Files in the newest version (43, still in beta) can send emails directly from context menu, that is, they incorporated the nautilus-sendto extension.
They use libportal, but libportal doesn't work well when I need to attach a file and I'm using Thunderbird.
I checked and xdg-email utility, from package xdg-utils, has some checks to use Thunderbird correctly. The idea is to implement to libportal too.

[nekohayo]

Presumably related: https://gitlab.freedesktop.org/xdg/xdg-utils/-/issues/177

Whole context: https://gitlab.gnome.org/GNOME/nautilus/-/issues/2431