racket/class: add missing check for #<unsafe-undefined>

When a superclass method is called before the superclass
is initialized, then all field accesses need to be guarded.

Robby found this one.
racket/draw and racket/snip: avoid some undefined-checking wrappers

The changes to `pen%` and `brush%` are related to the
`racket/class` repair of the previous commit, while the
`style%` change is because I didn't look at it before.
remove dependency on racket/snip (use file/convertible instead
of a direct reference to bitmap-snip%)
added pict-snip pkgs
added pict-snip to the main-distribution pkg
fix bug in names that begin with _r
add examples directly to judgment-holds docs

(as well as a pointer to other examples elsewhere)
use path->relative-string/library's cache argument to
make the 'Open Require Path...' dialog more responsive
meta/pkg-build: support version for pkg catalog query
fix broken doc link to R5RS

There's no good way to fix the link, right now. Since the Guide and
R5RS packages depend on each other, though, we can expect that they're
installed at the same time and a relative reference will generally work.
fix SRFI-62 link

Closes PR 14673
More titles and abstracts for RacketCon.
Add sponsors to RacketCon page.
rename t to tau
stop doing slow computation whose results were useless

also, clean up drawing of 'insert large letters' dialog
so that you don't see a little white sliver when there are
no characters typed
fix r6rs link just link in 8d3fd26c6
fix rule reference in tutoral, also spell-check

closes PR 14676
fix run-time error reporting for variables in a submodule

Error reports used the "source" field of a module, which
doesn't have submodule information, or the "name" field of
a module, which might not match an actual filename (".ss"
vs. ".rkt"). Create the right combination.
fix `rename-transformer-target` for chaperoned structs
fix "props" for "pict-snip-pkgs"