KFileItemModelRolesUpdater: Avoid refreshing items when we receive their content count

Merged Méven Car requested to merge (removed):KFileItemModelRoles-less-refresh into master

It concerns the detail view.

It can cause multiple preview to be generated for the same file. This is visible for directories whose icons use a random value for their inner files thumbnails.

Only the view needs to be updated for this model change.

cc @bruns @elvisangelaccio @alex

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