This patch changes the decorator to always apply stacking-order changes
immediately instead of deferring the re-stacking of the nitpicker views
to the next call of 'update_nitpicker_views'. The deferred application
did not always work when more then one windows changed their stacking
position at once because the cached '_neighbor' values interfered with
each other.
The eager re-stacking should not have negative effects on the user
experience because, in contrast to re-positioning, re-stacking a rare
operation.
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