Summary:
In general, Gwenview sorts directories first, then groups all archives
behind those and finally displays images. While the separation of images
worked fine when sorting by {nav Date}, directories and archives were
mistakenly mixed within a single group for that sorting order.

This is because the special handling for `KDirModel::ModifiedTime` in
`SortedDirModel::lessThan` does not take into account
`KDirSortFilterProxyModel::sortFoldersFirst()`. As directories cannot
come with a shooting date, we can delegate sorting to
`KDirSortFilterProxyModel::lessThan` which handles the separation of
directories just fine.

For `SemanticInfoDirModel::RatingRole` the problem could not be
observed, because  Gwenview did not pass on the rating of directories,
so both directories being compared had a zero rating and we fell back to
KDirSortFilterProxyModel::lessThan. Nevertheless, for clarity we make
the fallback for sorting by rating explicit too.

Depends on D13727

Test Plan:
In {nav Browse} mode, sort a directory containing directories, archives
and images by {nav Date} and check the correct grouping (also for
{nav Descending}).

Reviewers: #gwenview, muhlenpfordt

Reviewed By: #gwenview, muhlenpfordt

Subscribers: muhlenpfordt, faridb

Differential Revision: https://phabricator.kde.org/D13728