ensure that notifications are visible during animation
ensure that notifications are visible during animation

...which was enabled by bb9a4d77f32e632202028a7041b899d3536a8adf.
Animated actors are raised above everything in window_stack[],
but we want to see the notifications.

  * src/mcompositemanager.cpp (MCompositeManagerPrivate::fixZValues):
    If there's nothing mapped above notifications, place their actor
    above where the animation target(s) would be...
    -- MCompositeManagerPrivate::clientMessageEvent():
       ...but don't let the desktop follow, otherwise it could got
       above the iconifying window.

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