[Kickoff] Disable tabBar's mouseArea when searching

Summary:
When the root is in the "Search" state, we can safely disable it.

BUG: 401861

Reviewers: #plasma, hein, ngraham, davidedmundson

Reviewed By: #plasma, ngraham, davidedmundson

Subscribers: davidedmundson, ngraham, bugseforuns, dginovker, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28985