Use a searchbar similar to the one in okular using the webenginepart

Summary:
Similar to D19620 with khtml

Test Plan: Build and run

Reviewers: Konqueror, stefanocrocco, dfaure

Reviewed By: Konqueror, dfaure

Subscribers: dfaure, GB_2, ndavis

Maniphest Tasks: T10258

Differential Revision: https://phabricator.kde.org/D19622