Fix broken search in Documentation View

Summary:
The patch fixes broken search in Documentation View. Steps to reproduce:

1. Open Documentation View
2. Select CMake documentation
3. Type in the search line edit "set_"
4. Internal completer produce list of appropriate documentation items
5. With arrrow keys select "set_property" item (2nd) and press Enter
6. You will see "set_directory_properties" page - 1st element from completer list.

Such broken search happens every time - we will see the only first element from completer list.

New version fixes this annoying bug.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2937