Added focus for searchbar in history sidebar & bookmark sidebar

Summary:
After Adding Focus , user can directly search after pressing ctrl+h in history sidebar
& ctrl+shift+b in bookmark sidebar
BUG: 393948

Reviewers: Falkon, drosca

Reviewed By: Falkon, drosca

Subscribers: SGOrava, falkon, Falkon

Tags: Falkon

Differential Revision: https://phabricator.kde.org/D20022