IncrementalSearchBox: use autoFillBackground

Summary:
Resetting the widget palette to the app main palette fixes black
background problem in short and clean way.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: tcanabrava, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D14039