racket/gui cocoa: fix "&" in labels

As of Mac OS X 10.8, `setTitleWithMnemonic:` doesn't strip away "&".
file/convertible: declare 'png@2x-bytes conversion variant

The 'png@2x-bytes variant is like 'png-bytes, but where the decoded
bytes are intended to be scaled by 1/2.

Consumers:

 - DrRacket's print handler

 - Scribble's HTML renderer

Producers:

 - `bitmap%`s where the scaling factor is 2

 - picts

Examples: Quick docs, docs for `images/icons/misc`, DrRacket interactions
for results of `images/icons/misc` functions.
GC bitmap @2x
path-element? should disallow '.' and '..'

Fix `path-element?` to check whether the name returned by split-path
is a path or a special entry. This behavior matches both the documentation
of `path-element?` and the implementation of `string->path-element`.