User Story

As an developer, I need to handle the "onCheckSelectAll" event so that I can update the UI accordingly.

Acceptance Criteria

An event listener for onCheckSelectAll must be called with a CheckEvent.

Details

Listbox already posts the event, consider the same functionality:
https://github.com/zkoss/zk/blob/v9.6.0.1/zul/src/org/zkoss/zul/Listbox.java#L3876

Tree doesn't post the event:
https://github.com/zkoss/zk/blob/v9.6.0.1/zul/src/org/zkoss/zul/Tree.java#L2896

Also update the related javadocs to include the event.