An old hack was notifying MergedProxyModel users that max int rows had been
removed when a submodel was reset. This caused the code that invalidates
selected items to spin for a very long time. A modelAboutToBeReset signal,
introduced in Qt 4.6, allows notification before the submodel information is
lost.
Note: there is one other case, in RemoveSubModel, where this hack is used.
This is only called when a device is removed and doesn't trigger this
condition, but it should probably be addressed in the future.
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