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: {F5815497}

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