Fix DownloadDialog window margins to meet general theme margins

Summary:
Old code prevented the widget theme to use windowborder-specific margins for the
effective outer margins.

Test Plan: Content of Download dialog now has same margins to window border like other dialogs.

Reviewers: leinir

Reviewed By: leinir

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14095