Pass focus to child searchbar when KPluginSelector is focused

Summary:
This commit is a precursor to autofocusing on searchbar in
plasmasearch kcm module.

CCBUG: 399516

Reviewers: #plasma, #frameworks, ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D20034