Rename PaddedStyle to AbstractStyle

Make Style inherit AbstractStyle, so it becomes the base class
of all styles. AbstractStyle also now has a default "data"
property so it's possible to conveniently declare helper
content inside styles.

Change-Id: Id84f3762cfae506e65021322619d7fb77b137994
Reviewed-by: Tomasz Olszak <olszak.tomasz@gmail.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>