Set current tab as parent for duplicated tab

Summary: BUG: 408518

Test Plan: Duplicate tab, the change is visible in TreeView of Vertical Tabs plugin.

Reviewers: Falkon, drosca

Reviewed By: Falkon, drosca

Subscribers: falkon, drosca

Tags: Falkon

Differential Revision: https://phabricator.kde.org/D24933