fix keyboard focus problem in GridBrowserDelegate with an automatic qml test

Summary: fix keyboard focus handling in GridBrowserDelegate including an automatic test for it

Test Plan: the automatic test fails without the modifications of the delegate andd pass with them

Reviewers: Elisa, astippich

Reviewed By: Elisa, astippich

Subscribers: astippich

Differential Revision: https://phabricator.kde.org/D9970