KRearrangeColumnsProxyModel: reset in setSourceColumns()

Summary:
Otherwise the view won't update, when doing this dynamically.
With a test program.

Test Plan: See test program

Reviewers: kossebau, vkrause, davidedmundson, ahiemstra

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27876