The main button API only allows re-writing one color of the image.
Since we need to rewrite multiple colors for the switch, I introduced a
`image_batch` for buttons which lets you do all kinds of manual
fanciness, and pass the result into the button API.
data/ directory, because they're statically bundled; they're not
actually read from the filesystem. #253
Two SVGs get duplicated between widgetry and abst assets -- dont_walk
and arrow_drop_down.
