Cocoa: Fix crash on screen disconnect.

Maintain virtual siblings list on screen deletion.

QCocoaIntegration::updateScreens() has a loop which
will delete all non-current QScreen objects using
QPlatformIntegration::destroyScreen().

destroyScreen() vill eventually call QWindowPrivate::
setTopLevelScreen() which accesses the virtual siblings
list for the deleted screen.

This can cause a stale pointer access if the virtual
screen list is not up to date, especially when disconnecting
two screens at the same time.

Change-Id: Ia6b9d01edf8e5eea25b64604a2b3b28b173125f7
Task-number: QTBUG-48275
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>