Taskbar related class docs and overviews

Change-Id: Ib5aebea73f9637d62dc64f71050bb07b1998a66d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>