Update PLT sites
fix cross compile on QNX

got broken in 2e284cc78357ccc4a63aae7603682b0344bfc58f
The racket version of libunwind is not compatible with QNX but old-style stacktraces are still working with the default gcc version
Slight change to how drracket handles lexer/colorers, and doc changes to color:text<%>
add slide-pict/center
update dns tests based on change at nwu
Mac OS X PPC: fix dylib paths

Relevant to PR 14694
optimizer: more optimizations for unary operations

Previously, the optimizer simplified the application of some unary functions inside let,
for example (car (let () ... (cons 1 2)) => (let () ... 1). This commit extends this to begin forms,
like (car (begin ... (cons 1 2)) => (begin ... 1).

Also, constant folding and some reductions were only availed in the direct case, for example
(procedure? car) => #t. With this commit these reductions are extended to the expressions
inside let and begin, for example (procedure? (let () (begin ... car))) => (let () (begin ... #t).
RacketCon talks update.
Allow type annotations after for clauses in the for macros.

Cf. GH issue #751.
scribble/srcdoc: add `form-doc`
optimizer: transform ((begin ... proc) x) to (begin ... (proc x))

Currently the optimizer can convert ((let (...) ... proc) x) to
(let (...) ... (proc x)). This is useful especially if proc can be
inlined. Extend this to begin's forms.
optimizer: another ((begin ... proc) x) to (begin ... (proc x))

The tranformation this time applies before optimization of the rator
and complements Gustavo's variant, which applies after optimization of
the rator.
add an optional argument example to ->i docs
optimizer: Remove unused flag added in d14b4a8
racket/gui: change placement of children in `panel%` and `pane%`

The `panel%` and `pane%` classes were not originally intended for
direct instantiation; instead, `horizontal-pane[l]%` and
`vertical-pane[l]%` provide basic placements that make sense for
multiple children. Adjusting `pane[l]%` to just overlay all children
seems both sensible and useful (if only one child is shown at a time,
for example).

As suggested by David Nelson.
RacketCon schedule.