Cocoa: Make sure child windows' QNSView is properly released

Child QCocoaWindows don't have an assigned NSWindow, but still get a
QNSView as backing store. That QNSView is added as a subview to
the parent window's QNSView. That would leave an unmanaged QNSView
alive.

Change-Id: I10f57fb971f7aa1e278bb0af4217052d1e4bb0bc
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -214,6 +214,8 @@ QCocoaWindow::~QCocoaWindow()
 
     QCocoaAutoReleasePool pool;
     clearNSWindow(m_nsWindow);
+    if (parent())
+        [m_contentView removeFromSuperview];
     [m_contentView release];
     [m_nsWindow release];
     [m_nsWindowDelegate release];