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
2 jobs for !15 with sixel in 1 minute and 39 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #11809
build

00:00:40

 
  Distcheck
passed #11810
distcheck

00:00:59