Add new instance button to tabwidget

Summary:
set the corner widet to a button connected to newAtCoreInstance.

when a new atcoreinstance is created its tab will be set to current

Reviewers: tcanabrava, patrickelectric, laysrodrigues

Reviewed By: laysrodrigues

Subscribers: Atelier: General Development

Tags: Atelier: General Development

Differential Revision: https://phabricator.kde.org/D12011