Pressing Enter should open submenus in Kicker

Summary: BUG: 375756

Test Plan:
Pressed enter on an item with children. Menu appeared
Pressed enter on an application. It launched as before

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: hein, mart, cfeck, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4401