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)
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