Similar to start a test thread, start a test task.

This should be a hint to consider later if thread/task overview widget will be implemented.
Add possibility to observe any worker/thread/job.

Add Activity widget.
Adjust thread sample.
Change activity progress text to CellRendererProgress
Job name and description.
Allow also Tasks to observe, add Test in ThreadWorkerWidget.

Overwork JobInformation a bit.
Adjust ThreadWorkerWidget gui
Activity widget can cancel tasks if cancellation is supported
Add tree view row step by step to avoid some GTK warnings
Add cancellation support for started tasks.
Message window scroll automatically to new added text at end