Overhaul the TaskbarButton QML type

Make the overlay a grouped property, and use QQuickIconLoader
internally to support remote icon sources.

Change-Id: I888244fe5384702dceebe372b0bd0dd50891629a
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>