Add support for displaying total number of groups in subgroups in Gnus

Add support for displaying total number of groups in subgroups in Gnus

* lisp/gnus/gnus-topic.el (gnus-topic-line-format)
(gnus-topic-line-format-alist, gnus-topic-prepare-topic)
(gnus-topic-insert-topic-line, )
(gnus-topic-display-missing-topic, )
(gnus-topic-update-topic-line):
Provide number of groups in subtopics for topic line display and
add 'G' element to be used for 'gnus-topic-line-format' variable

* doc/misc/gnus.texi (Topic Variables):
Document 'G' element for 'gnus-topic-line-format' variable (bug#49068).