[applets/taskmanager] Make launcher item tooltips look identical to panel widget tooltips

Test Plan:
Before: {F8095730}

After: {F8095728}

Reviewers: #vdg, #plasma, ndavis

Reviewed By: #vdg, ndavis

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T12640

Differential Revision: https://phabricator.kde.org/D27301