New feature: running task widget

Summary:
Start a task and a small line appears at the top of your desktop.
Moving the mouse onto that line expands to a bar that shows

[Stop]  Task title   [Done],

as a reminder of what the current task is.

Design (new classes) :

• RunningTaskModel to store the running task (with base interface for type safety of stubs).
• RunningTaskWidget and PageView on top of that model
• TaskApplicationModel to create the RunningTaskModel with its deps
• TaskApplicationComponents to connect PageView and RunningTaskWidget to RunningTaskModel

Test Plan:
No cucumber tests because OpenSuSE botched the cucumber packaging.

(https://bugzilla.opensuse.org/show_bug.cgi?id=1015940)

Unit tests for all new classes though, and for serialization of the 'running' attribute.

Reviewers: franckarrecot, ervin

Reviewed By: ervin

Subscribers: Zanshin

Differential Revision: https://phabricator.kde.org/D3032