Avoid calling KFileItemModel::index() in KFileItemModelRolesUpdater

Avoid calling KFileItemModel::index() in KFileItemModelRolesUpdater

KFileItemModel allows to find out the index of a KFileItem with its
index(const KFileItem&) method. Calling this method is not extremely
expensive, but it's also not free (it looks up the URL of the KFileItem
in a QHash, i.e., it has to call qHash(QString) for the full URL).

In KFileItemModelRolesUpdater, we sometimes converted the known index to
a KFileItem and then back to an index in applyResolvedRoles(KFileItem).
This patch fixes this by modifying applyResolvedRoles such that it takes
the index directly as its argument.

REVIEW: 114847