(reftex-section-levels): Allow negative levels.

(reftex-section-levels): Allow negative levels.
(reftex-section-info): Handle negative section levels.
(reftex-view-crossref-extra): New option.
(reftex-view-crossref): Generalized to work in more places.
(reftex-find-citation-regexp-format, reftex-find-reference-format,
reftex-macros-with-labels, reftex-global-search-marker): New variables.
(reftex-view-cite-locations-from-bibtex): New command.
(reftex-view-regexp-match, reftex-global-search,
reftex-global-search-continue): New functions.
(reftex-extract-bib-entries): No error for non-existing BibTeX
file, just a message.
(reftex-extract-bib-entries): Better error handling.
(reftex-select-with-char): Use `*RefTeX Select*' for selection
help and force it on current frame.
(reftex-default-bibliography): New function.
(reftex-access-parse-file): Check consistency after loading a
parse file.  When inconsistent, enfore rescan.
(reftex-check-parse-consistency): New function.
(easy-menu-define): Options section drastically shortened.