Cleanup properly when destroying QCocoaWindow

[NSNotificationCenter addObserver]'s documentation clearly states that
removeObserver needs to be called before releasing the object being
observed. The m_contentView we create was never released either, so be
sure to release that as well.

Change-Id: Ia54eb1c5c751f4cb0edb21ad559b261cb8f24208
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -123,6 +123,8 @@ QCocoaWindow::QCocoaWindow(QWindow *tlw)
 
 QCocoaWindow::~QCocoaWindow()
 {
+    [[NSNotificationCenter defaultCenter] removeObserver:m_contentView];
+    [m_contentView release];
     [m_nsWindow release];
 }