Add possibility to sort by descending order

Summary:
Gwenview only allows sorting by ascending order. This patch adds the possibility to switch the sorting order to descending.
It is implemented similarly to the sorting menu in Dolphin.
The sorting order is also saved into the Gwenview configuration file.

Before:

After:

FEATURE: 293333

Test Plan:

• Open a folder which contains images in Gwenview
• Check 'Descending' in View > Sort By
• The sorting order will be switched to descending
• Uncheck 'Descending' in View > Sort By
• The sorting order will be reverted to ascending

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: muhlenpfordt, rkflx, xyquadrat

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D13213