onToolbarButtonClick: function(event) {
        if(event.button == 0){
            // left click
            gbrain.toggle(0);
        }else if(event.button == 1){
            // middle click
            gbrain.openAndReuseOneTabPerURL('http://www.google.com/bookmarks/lookup?q=label:!gbrain');
        }
        // right click not handled here, context action used instead
    },