FileChooser portal: add support for opening directories

Summary: BUG: 419874

Test Plan: This is also implemented on Qt side and currently on review: https://codereview.qt-project.org/c/qt/qtbase/+/297872. Tested with xdg-portal-test-kde using mentioned Qt patch.

Reviewers: apol, #plasma

Reviewed By: apol

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29071