Style border of Open UI fields

The OpenUiString field has different styling in Wicket and AngularJs. In Wicket the field is surrounded by a box and 8px margin. In AngularJs the frame is not styled at all.

The styling should be fixed in such a way that

1. the field border looks good in both editors
2. the extension can assume zero padding/margin of the html and body elements (i.e. it's contents touches the frame border directly) so it has full control over any additional margin and padding.
3. the extension still looks OK when it shows a scroll bar (if it wants that for some reason)