WIP: Sixel support

Very much WIP (just needed some distraction and thought how hard can it be), but it at least works with mpv's vo_sixel to play videos in the terminal.

If someone else wants to pick it up, or just implement it themselves, feel free, not sure if I'm going to finish this. The biggest issue left is probably that it doesn't clean up images ever. It should be easy to do it efficiently, by doing something like adding a LINE_Sixel line attribute, and doing garbage collection when clearing history. But that probably involves moving the image storage to Session instead of Screen or something.

Link to the related bug so it isn't forgotten: https://bugs.kde.org/show_bug.cgi?id=391781