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