Fix layout in DownloadItemsSheet

Summary:
The previous code was primarily there to work around misfeatures of OverlaySheet which no longer exist. Consequently, we don't have to leave in the hacks. Nice.

This further builds on the new OverlaySheet work done by Marco in https://commits.kde.org/kirigami/6f3800979f1387955ebd9c12aac031a9704853ae - resulting in the whole thing working much better, and with less client code. Nice.

BUG:419535

Reviewers: Frameworks, bugseforuns, mart, Plasma, KNewStuff, ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28540