racket/gui cocoa: preserve resolution of bitmaps as control labels

This change is mainly for Retina display mode.
racket/draw: add a backing-scale argument to bitmap constructors

Generalizes backing-scale support created for `make-platform-bitmap`
and Mac OS X in Retina mode so that any bitmap can be created with
a backing scale (except monochrome bitmaps or bitmaps with masks).
images/icons: add `backing-scale` arguments

By default, icons render with a backing scale of 2.
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.
Removed recently broken "doc-tests.rkt"

The breakage was from `declare-exporting' now requiring at least one
module path. Plot's docs build makes the test superfluous, anyway.
Racket splash/about logo @2x
Dienprogramm logo @2x
if a bitmap has a non-1 backing scale, always draw smoothly to other contexts
Make tc-setup just take a function instead of 3 identifiers.
Split out do-standard-inits.
Replace init argument with direct calls to do-standard-inits.
Add tc-toplevel/full and tc-module/full and use them.
Remove info file from test directory.
Document alternative type annotation form
Export `unsave-bug-report`. Fix doc typo.
Minor doc fixes.
Provide and document `match-expander?` and `legacy-match-expander?`.