[Notifications KCM] Fix keyboard navigation in apps list

Summary:
Learned how to do this from @guoyunhe in D24798!

BUG: 409024
FIXED-IN: 5.17.4

Test Plan:
1. Click on list item -> list gets focus, item is shown
2. Use up and down arrow keys -> items are shown
3. Click on search field and tab into list -> you can select things with the keyboard
4. Search for something and click on or tab into apps list -> the correct item is shown

Reviewers: broulik, #plasma

Subscribers: plasma-devel, guoyunhe

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25309