The menu now used toggle and radio for some items.

The menu now used toggle and radio for some items.
(reftex-default-context-regexps): `caption' now prefers the
optional short caption.
(reftex-offer-label-menu): Fixed bug which could kill master
buffer of external document.
(reftex-select-item,reftex-get-buffer-visiting): Compatibility
code works now the other way round.
(reftex-select-external-document): Now gives a message when no
external documents are available.
(reftex-find-duplicate-labels): Single key strokes to exit or to
do a query replace.  Made more user friendly in general.
(reftex-section-levels,reftex-default-context-regexps): Move
definition of these variables to configuration section.