Misc changes in gnus-group.el and gnus-topic.el

Misc changes in gnus-group.el and gnus-topic.el

* gnus-group.el (gnus-tmp-*): Declare.
(gnus-update-group-mark-positions): Remove unused `topic' var.
(gnus-group-insert-group-line): Remove unused var `header'.
(gnus-group--setup-tool-bar-update): New function.
(gnus-group-insert-group-line): Use it.
(gnus-group-update-eval-form): Declare local
dynamically-bound variables.
(gnus-group-unsubscribe-group): Use \` and \' to match string bounds.

* gnus-topic.el (gnus-topic-jump-to-topic)
(gnus-group-prepare-topics, gnus-topic-update-topic)
(gnus-topic-change-level, gnus-topic-catchup-articles)
(gnus-topic-remove-group, gnus-topic-delete, gnus-topic-indent):
Use inhibit-read-only.
(gnus-topic-prepare-topic): Use gnus-group--setup-tool-bar-update.
(gnus-topic-mode): Use define-minor-mode and derived-mode-p.