Commit 7781d6ef broke sorting in the sense that folders are no
longer sorted before files. This is due to KRecursiveFilterProxyModel's
implementation of sort() being used instead of ArchiveModel's.

We now subclass KRecursiveFilterProxyModel into ArchiveFilterSortModel
so we can re-implement lessThan() and move the relevant code from
ArchiveModel::sort().

Differential Revision: D3742