This implements as described at https://phabricator.kde.org/T12793
Modifications have been made to both kdecoration and kstyle so that window decorations for both normal windows and MDI applications will be affected, with new designs placed in libbreezecommon as are shared between both. A configuration in the Breeze window decoration settings will affect both kdecoration and kstyle so .kcfg files in each have been updated. 'Oxygen' remains the default style as at present.