e4x: Cope with E4X removal
Added support for 3.9.x

I had this locally for a looong time it's nor really fair not to push
these changes even when available time is sparse.

As soon as 3.9.92 is released I'll upload for review as the review queue
is really long already, but I'm pretty sure the extension will be
rejected if we mention we support 3.9.92 but it's not even released yet.
Enable back transparency

This is a huge hack, don't do this at home kids! More seriously, I'm
quite shocked that the feature will be removed in VTE itself, that means
all Gtk terminals will lose this feature.

Removing it from gnome-terminal only would have been more acceptable to
me and the rationale is low (as a developer, I'd have prefered the
honesty to say the maintainer does not care at all).

See https://bugzilla.gnome.org/show_bug.cgi?id=708422 for the final
resolution.