Filepicker reads thumbs preview from Dolphin settings

Summary:
Open/Save dialog uses the settings from Dolphin to determine what to show in the thumbnails.
Also now supports much more file types.

BUG: 318493

Test Plan:

Reviewers: Frameworks, VDG, ngraham, Dolphin

Reviewed By: VDG, ngraham

Subscribers: elvisangelaccio, alexeymin, ngraham, Frameworks

Tags: Frameworks

Maniphest Tasks: T8552

Differential Revision: https://phabricator.kde.org/D12389