racket/draw: add `#:try-@2x?` argument to `read-bitmap`
DrRacket: enable @2x bitmaps for tool icons
some @2x tool icons
racket-style.css: move margin-note img formatting to Guide/Ref icon

Setting image with to 1.5em seems to be aimed specifically at
the cross-reference icons in the Guide and Reference, and the
width should not apply to just any image in a margin note.