Move `weechat-nick-action' from button to weechat.el; use it in speedbar.

The function `weechat-nick-action' is not button specific and useful
in speedbar.  Maybe it should be moved to a weechat-util.el or similar
instead to make weechat.el smaller.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>