Fixed: wrong option for the observer in _CPPopoverWindow

Previously the option of the observer in _CPPopoverWindow were set to 0, now it's set to CPKeyValueObservingOptionNew.

This PR fix also another issue, the popoverWindow registered the observer in the wrong order. It firstly observed the frame of the sender and then removed it directly. Now it register when ordering front the popover and remove it when closing the popover
Fixed: changed option for observer in _CPPopoverWindow.j
Merge pull request #2276 from Dogild/PopoverWindowObserver

Fixed: wrong option for the observer in _CPPopoverWindow