D8232: Add option to center shadow

[rodrigo pelorosso]

rpelorosso added a reviewer: Breeze.

REPOSITORY
  R31 Breeze

REVISION DETAIL
  https://phabricator.kde.org/D8232

To: rpelorosso, #breeze
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart