static/ext/src/toolbar/Paging.js
function(){
        var me = this,
            prev = me.store.currentPage - 1;

        if (prev > 0) {
            if (me.fireEvent('beforechange', me, prev) !== false) {
                me.store.previousPage();
            }
        }
    }
function(){
        var me = this,
            prev = me.store.currentPage - 1;

        if (prev > 0) {
            if (me.fireEvent('beforechange', me, prev) !== false) {
                me.store.previousPage();
            }
        }
    }
Move to the previous page, has the same effect as clicking the 'previous' button.