D8362: added setting to toggle drawing of title bar separator

[Emirald Mateli]

emateli added a reviewer: Breeze.

REPOSITORY
  R31 Breeze

REVISION DETAIL
  https://phabricator.kde.org/D8362

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