function _onClickBtnWatch(evt){
            var button = $(evt.target);
            var watching = button.data("watching");
            var url = watching ? vars.urls.unwatch : vars.urls.watch;

            $.post(url, function(){
                button.data("watching", !watching)
                    .toggleClass('ybtn-watching')
                    .html(Messages(!watching ? "issue.unwatch" : "issue.watch"))
                    .blur();

                $yobi.notify(Messages(watching ? "issue.unwatch.start" : "issue.watch.start"), 3000);
            });
        }
"click" event handler of watch/unwatch button.
Toggles watch/unwatch issue.

@param evt
@private