Don't accept QWSI::CloseEvent when the window is blocked by a modal window

If a window is blocked by another modal window, and we decide that as
a result we should not propagate the close event, we need to also
report this back to the platform plugin, so that it can tell the
operating system to not close the window.

This is a problem on macOS, where the system doesn't natively support
transient parents being blocked by a window-modal window, so the OS
will still allow interaction with the title bar close button, resulting
in a QWSI close event being delivered.

Fixes: QTBUG-104905
Pick-to: 6.5 6.2
Change-Id: I09ff15b0fbb1002a8f9d83d932ca766ed510f0a0
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/gui/kernel/qguiapplication.cpp

@@ -2706,6 +2706,7 @@ void QGuiApplicationPrivate::processCloseEvent(QWindowSystemInterfacePrivate::Cl
     if (e->window.data()->d_func()->blockedByModalWindow && !e->window.data()->d_func()->inClose) {
         // a modal window is blocking this window, don't allow close events through, unless they
         // originate from a call to QWindow::close.
+        e->eventAccepted = false;
         return;
     }