support popup position option: at pointer inside the reference component

User Story

As an application developer, I want to show a popup/menupopup at the position a user's mouse pointer like at_pointer, but is always inside the reference component even a user move his mouse out of the component's boundary, so that the popup/menupopup doesn't show at a position out of the reference component.

Simulated case

run the attached zul, right click the listitem and move the mouse pointer immediately, because the client is busy, so the menupopu will show at the last position of the mouse, not the clicking position.

Acceptance Criteria

Details

Put document or specification reference