Add support for vt240(or +) for sixel support

[wagoodman]

With vt240 (or higher) support for sixels would allow for utilities like https://github.com/hackerb9/lsix (this sounds like it could be a pretty heavy lift given that this is dependent on vte, which I think doesn't support anything above vt220)

[gnunn1]

This would need to be done in VTE and they all have an issue open for sixel support in their issue list.

[algj]

As far as I can see - it's now possible to build VTE with sixel support.

Related issue: mate-desktop/mate-terminal#410

Mate-terminal is using vte_terminal_set_enable_sixel if (vte_get_feature_flags() & VTE_FEATURE_FLAG_SIXEL) != 0:
https://gitlab.gnome.org/GNOME/gnome-terminal/-/blob/master/src/terminal-screen.cc#L1541-1543

Though, vte does not enable sixel support by default unfortunately, you need to build it yourself and enable it from your terminal.

https://gitlab.gnome.org/GNOME/vte/blob/master/src/vtegtk.cc#L7944-7958

It appears it is not compiled by default with sixel support because they haven't fixed some of the issues, I'll test it out and let everyone know how well it works on Tilix.

Update:

Well, I compiled VTE with sixel support. I added the sixel support on tilix:

algj@c42e569

For some reason I failed to make sixels display, same issue appears on vte terminal itself too, just an empty space instead of the image. I think it may not work on GTK4...?