Introduce a toolbar for Log Edit mode

Introduce a toolbar for Log Edit mode

* etc/NEWS: Announce new change.

* etc/images/README: List the sources of the new image files.

* etc/images/commit.xpm:
* etc/images/commit.pbm:
* etc/images/gen-changelog.xpm:
* etc/images/gen-changelog.pbm:
* etc/images/ins-changelog.xpm:
* etc/images/ins-changelog.pbm:
* etc/images/load-changelog.xpm:
* etc/images/load-changelog.pbm:
* etc/images/view-diff.xpm: New files.

* lisp/vc/log-edit.el (log-edit-menu): Insert a menu item for
"Generate ChangeLog".
(log-edit-tool-bar-map): New keymap.
(log-edit-mode): Install this keymap as the tool bar map.

(cherry picked from commit b7fc3ab787d8dd926e1912e51e99f0efcf57cb02)