Fix ignoring close events on OSX

QNSWindowDelegate was not handling windowShouldClose, which is how you
can tell Cocoa that your window should not close if the close button is
pressed. This change moves the close handling from windowWillClose to
windowShouldClose, and adds an optional "accepted" pointer to
QWindowSystemInterface::handleCloseEvent so that QNSWindowDelegate can
return a true/false value for whether the window should actually close

Task-number: QTBUG-28965
Change-Id: I67c6296ad42cbeeb71413e05411467d4e558adb4