Instanciate the launcher tasks model only when needed.

Summary:
Previously the launcher tasks model was always instanciated; with
this change it's only done once a launcher list gets set or a
request to add a launcher happens. This avoids an unnecessary model
in TasksModel instances that will never store any launchers (e.g.
in the Window List applet).

Reviewers: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2378