Add datebox attribute to enable or disable close on timeZone change

User Story

https://zkfiddle.org/sample/1g459qh/1-Another-new-ZK-fiddle

When selecting the datebox timezone in the datebox popup, the popup will always close after changing the the timezone.

This generates additional actions for the end-user to return to the popup then select the date.

Acceptance Criteria

Simplify the user workflow by allowing the popup to stay open when the user selects a timezone.

Create a new attribute on the datebox "closePopupOnTimezoneChange" or similar to control this behavior.

Default behavior should be close on onTimezoneChange for compatibility with existing applications.

Details

https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/db/Datebox.js#L541