This was way too hard!

We really need to reimplement in-browser view source mode. But we have
always supported external view source mode, using the default text/plain
handler, via a hidden GSetting. So even once we restore in-browser view
source, we will still want to keep this code around.

We have to use the portal D-Bus API manually because the only way to get
it using GLib is to use g_app_info_launch_default_for_uri(), and that
will always pick Epiphany itself as the default handler, because
Epiphany supports opening HTML files. The portal is only accessible via
a fallback path.

https://bugzilla.gnome.org/show_bug.cgi?id=787849