RunningTaskModel: update shown title when renaming the currently running task

Summary: This was missing, so the old title was always shown in the running-task-widget.

Test Plan:
Start a task, rename it zanshin (either in the listview or in the editorview),
the running-task-widget now shows the new title.

Reviewers: ervin

Subscribers: Zanshin

Differential Revision: https://phabricator.kde.org/D12634