D8361: Add option to change Oxygen's windows decoration shadow intensity

[rodrigo pelorosso]

rpelorosso added a reviewer: VDG.

REPOSITORY
  R113 Oxygen Theme

REVISION DETAIL
  https://phabricator.kde.org/D8361

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