Fix invalid reference of toolTipArea.

Summary:
When activateTask is called from root Item, it will fail to resolve the
reference of toolTipArea. This change make toolTipArea a property of Task
and make activateTask() access it via this property.

Test Plan: Manually test via Meta+number on group item.

Reviewers: hein, Plasma, mart

Reviewed By: mart

Subscribers: mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4996