We should not forget the launchers that are not currently shown

Summary:
The pinned launchers that are assigned to other activities
will have a -1 as the order. We should not forget them
when saving to the configuration file.

Reviewers: hein, mart

Reviewed By: hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5933