make color of table list heading inherit color

• the grey background color of the normal admin ui-list-headrows doesn't make sense
  when used in a detail widget
add class “reset_padding” for resetting panels

• often a accordion panel needs no padding, f.e. for including list-widget contents
  so just add this class to reset the default behaviour
move error_display styles

• in order to be available in all contexts
• cleanup