BorderLayout slide action server-side support

In the current implementation, BorderRegion will slide in and out when clicked (unless clicking on the "open" bracket icon).

This happens on client side only. No server event (beyond onClick) is sent. The BorderRegion components (East, ...) do not have a method to slide in or out.

Possible improvement:

• Have a possible listener for "onSlide" {slide:boolean}

  similar to "onOpen"

  {open:boolean}
• Have a BorderLayout method similar to setOpen() / isOpen() to get or set the slide status like: setSlide() / isSlide()