Draft: terminal: add support for Sixel images
VTE introduced Sixel image support around 0.61.90. Information about Sixel can be found here https://en.wikipedia.org/wiki/Sixel
This commit leverages VTE's support for displaying Sixel images to enable it in xfce4-terminal when the appropriate conditions (built with a new enough VTE, distro VTE is compiled with sixel support) are present.
Addresses #76