Cleanup use of TasksModel data roles.

Summary:
Avoid a wall of prop rebindings to access roles outside the delegates
causing unnecessary calls to data().

Be 'typesafe' about testing against true. If a role returns QVariant(),
just binding it to e.g. a 'visible' prop still evaluates to true
incorrectly otherwise.

Reviewers: broulik, dfaure

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1934