Show decoration elements without frame in all widget styles

The box containing decoration elements was given a styled panel frame since the code was introduced in unmaintained/kdepim@22c8fd7a.

But IMHO things just look strange, when styles actually draw a frame for that option (current Breeze & Oxygen draw nothing). When things are properly aligned, the human brain and eye will properly get relations between the elements on the UI, any frames to help here just add visual noise. Same like the date label also is not getting a frame of its own.

Ignore the bad alignment and content of the decoration elements in the screenshots below, working on that separately.

Plastique style (before):

Windows style (before):

Plastique style (after):

Windows style (after):

For comparison, Breeze (no visual change here):

@winterz @gditchfield @mlaurent