Added support for descriptions in the group buffer:

Added support for descriptions in the group buffer:
(gnus-group-prepare, gnus-group-prepare-line, gnus-setup-news)
(gnus-gnus-to-quick-newsrc-format, gnus-group-list-groups):
Record if we show all groups, to redisplay
correctly after toggling descriptions display.
(gnus-newsgroups-display-toggle, gnus-newsgroups-update-description)
(gnus-newsgroups-retrieve-description): New functions.
(gnus-group-mode-map): Bind t to gnus-newsgroups-display-toggle.
Add a newsgroups-update-description menu.
(gnus-newsgroups-regex, gnus-newsgroups-display, gnus-newsgroups-alist)
(gnus-newsgroups-hashtb, gnus-newsgroups-showall): New variables.
(gnus-group-group-name):  Changed the regexp to avoid  conflict
with descriptions which have a ':' inside.
(gnus-group-mode): Doc fix.