Make mapping between sorting enum and model columns explicit

Summary:
In Gwenview sorting is implemented in SortedDirModel::lessThan, which
relies on setting the correct sort column to sort by different criteria
when calling sort in BrowseMainPage::updateSortOrder. In
gwenviewrc and in View → Sort By the Sorting enum is used,
while SortedDirModel has KDirModel::ModelColumns.

Previously mapping between both enums was done implicitly, which
worked because both order and position of the entries matched. However,
this had some caveats:

• Changes to KDirModel could invalidate Gwenview's configuration file, since the mapping might fail afterwards.
• Adding another sort criterion like Rating in D13344 would result in it mapping to the KDirModel::Permissions column, which could lead to surprising sorting results.

By making the mapping explicit and also providing a way to set a sort
role, both potential issues can be avoided.

Test Plan: All options in View → Sort By should work as before.

Reviewers: Gwenview, faridb, muhlenpfordt

Reviewed By: Gwenview, muhlenpfordt

Differential Revision: https://phabricator.kde.org/D13669