KFileItemModelRolesUpdater: waste less ressources and fix some bugs

KFileItemModelRolesUpdater: waste less ressources and fix some bugs

The main change in this commit is that we do not determine expensive
roles (like previews, mime types, etc) for all items, but only for the
visible ones and those close to the visible area or on the first and
the last page of the view.

This prevents that the CPU and hard drive are kept busy for quite some
time after entering a folder while all items are handled asynchronously.

There is one known problem at the moment: when sorting by "Type" or
another role that can be resolved by KFileItemModelRolesUpdater, the
icons of the visible items are sometimes not loaded while the sorting is
still in progress. I will try to fix this issue during the next few
days.

REVIEW: 110839