Enable showing top level items in kicker and kickoff menu

Summary: This is follow up fix for [[ https://phabricator.kde.org/D8816 | D8816 ]]. It enables showing root items in kicker and kickoff menus but not in dash.

Reviewers: hein

Reviewed By: hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10938