Fix manually reordering launchers.

Summary:
syncLaunchers() is called after drag operations end to write the order
of launchers in the all-capping TasksModel to the LauncherTaskModel
source model. The per-activity launcher pinning work recently broke
this by changing the model that's looped over.

Reviewers: #plasma, ivan, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4608