[dolphin/search] Add missing parsing for type 'Folder'

Summary:
Add missing parsing for type 'Folder'
See D24422

Test Plan:
dolphin --new-window 'baloosearch:?json=%7B%22type%22:[%22Folder%22]%7D'
- `Folders` is selected in search options

Reviewers: #dolphin, elvisangelaccio, meven, ngraham

Reviewed By: #dolphin, ngraham

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D24448