Start an interval task to continuously poll all the fields in the form for changes in their
values. This is normally started automatically by setting the {@link #pollForChanges} config.
@param {Number} interval The interval in milliseconds at which the check should run.
function(interval) {
        this.stopPolling();
        var task = new Ext.util.TaskRunner(interval);
        task.start({
            interval: 0,
            run: this.checkChange,
            scope: this
        });
        this.pollTask = task;
    }