Enable preview by default in the filepicker dialog

Summary:
Enable icon thumbnail preview in the Open/Save dialog by default.
This also enables it for users who previously disabled it.

Because of D12321: Hide file preview when icon is too small, it will only appear where it will actually be useful.

Depends on D12321

Test Plan:

• Open any filepicker dialog
• Preview will be enabled

Reviewers: Frameworks, VDG, rkflx, ngraham

Reviewed By: VDG, rkflx, ngraham

Subscribers: elvisangelaccio, abetts, rkflx, ngraham, Frameworks

Tags: Frameworks

Maniphest Tasks: T8552

Differential Revision: https://phabricator.kde.org/D12328