Cocoa integration - fix QCocoaWindow::setContentView

- QCocoaWindow::setContentView calls -removeFromSuperView
and this is not valid for a view that is a content view for a NSWindow:
using it will release the view but not clear the window.contentView pointer.
Set contentView to nil instead.

Fixing this makes visible the second problem:

- QNSWindowHelper in its -handleWindowEvent: can access content view and assumes
it's QNSView - it is not always guaranteed and can result in invalid message
sent to a view, we can use m_qtView instead (it will be nil if it
has a type different from QNSView, the call will be noop then).

Task-number: QTBUG-53325
Change-Id: I0472eba8165a04b6a3f81b2171b3bb9827ff5681
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -148,7 +148,7 @@ static bool isMouseEvent(NSEvent *ev)
         if (NSMouseInRect(loc, windowFrame, NO) &&
             !NSMouseInRect(loc, contentFrame, NO))
         {
-            QNSView *contentView = (QNSView *)pw->contentView();
+            QNSView *contentView = pw->m_qtView;
             [contentView handleFrameStrutMouseEvent: theEvent];
         }
     }
@@ -1173,7 +1173,11 @@ NSView *QCocoaWindow::contentView() const
 void QCocoaWindow::setContentView(NSView *contentView)
 {
     // Remove and release the previous content view
-    [m_contentView removeFromSuperview];
+    if (m_nsWindow)
+        [m_nsWindow setContentView:nil];
+    else
+        [m_contentView removeFromSuperview];
+
     [m_contentView release];
 
     // Insert and retain the new content view