Let kicker display executable items instead of empty buttons.

Summary:
This would partially fix https://bugs.kde.org/show_bug.cgi?id=369242
Another patch is required not to show empty elements (in case `data()` returns `QVariant()`).

Reviewers: hein, #plasma, davidedmundson, mart

Reviewed By: #plasma, davidedmundson, mart

Subscribers: mart, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2846