1. Introduce image UUIDs, so nodes may check for source updates.
2. Flip over design. Now, CHiPubNodes contain private data, instead of
CHiPubNodes being contained within private data. Each node must cache
its source data if it wants to conditionally run.
screen_capture_lite turned out to be pretty broken and so I brought back
my old X11 implementation for the Window node, for Unices only.
Hopefully SCL actually works on Windows because lemme tell you, I do not
want to go knee-deep in that.
Additionally, SAIL was replaced with stb_image because I couldn't get
SAIL to build under MinGW.
