convert setDirLister to the new connect syntax

Summary:
The only tricky part is the use of QOverload<>::of to distinguish between signals with same name but different signatures.

Test Plan: kdirmodeltest passes all tests.

Reviewers: dfaure, Frameworks, apol

Reviewed By: apol

Subscribers: apol

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12095