Resurrect background image support

[realh]

RoxTerm used to support background image.

Even though libvte-2.91 (vte tarball >= 0.38) no longer explicitly supports background image, it is possible to work it around with a bit of Gtk+ magic, by drawing behind a transparent (or translucent) vte widget.

See the Terminix project which has already implemented this.

(Originally reported by @egmontkob)

[realh]

(Seems to me that feature requests are not yet migrated to github?)

So, both Tilix (formerly Terminix) and xfce4-terminal have implemented background image support using some GTK+ magic.

Moreover, vte-0.52 introduces a new API vte_terminal_set_clear_background() which is intended to somehow make it easier.

Don't ask about the details, I'm not familiar with them, sorry.

-@egmontkob

[realh]

(Seems to me that feature requests are not yet migrated to github?)
I tried to migrate them too, but some didn't make it over for some reason.

My thoughts: CSS would probably make it easier, but I'm not sure whether I'd have to add it to the VteTerminal or to the widget behind it to enable the image to be faded into the background.

The Appearance page in the profile editor is already full-looking, I think I should make a separate "Background" tab, which I think it used to have in the past when the original background image option was still supported.

If VTE 0.52 is going to make it easier I'll wait until then.