Register AbstractTasksModel as uncreatable type.

Summary:
This makes the AdditionalRoles enum available to Qt Quick. Useful for
calling data() TasksModel from inside QML, which uses AbstractTasksModel
roles but doesn't actually inherit from AbstractTasksModel.

The reason TasksModel doesn't inherit from AbstractTasksModel is
because it inherits from QSortFilterProxyModel, and it doesn't fly
to multiple-inherit from two QObject-derived classes. This is also
why AdditionalRoles can't be moved to AbstractTasksModelIface easily.

The VDG is working on tooltip changes that require this.

Reviewers: Plasma, davidedmundson, subdiff

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3641