Documentation toolview: support incremental search on typing in search bar

Summary:
Search-while-typing and matches user expectations some more,
given behaviour in other current software.

Reviewers: KDevelop, brauch

Reviewed By: KDevelop, brauch

Subscribers: brauch, igorkushnir, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D7061