Fixed: popoverWillShow called when the popover is opened

Previously, the delegate popoverWillShow was called even if the popover was already shown. This PR fix this issue.
Also, it was possible to launch several times the method orderFront of the _CPPopoverWindow from the CPPopover.
For example with a double click on a button. Now the _CPPopoverWindow has a var isOpening and nothing else is possible during the opening of the popover (not possible to close or to click somewhere else etc etc). This work as in Cocoa.

Test app in Tests/Manual/CPPopover/