Documentation: small UI fix

Summary:
This patch fix Return key catching during documentation search. Current version process this key pressing only when user select some item in popup list. If no item is selected, then Retrun key does nothing:

1. Open CMake documentation
2. Type set in the search line edit.
3. Completer will show popup with set keyword at beginning.
4. Press Return - nothing happens.

It seems to be right to use first popup item as active item when Return is pressed.

Test Plan: Tested on master branch.

Reviewers: KDevelop

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3255