D5128: Set no stepper button for GTK3 scrollbars on gtk >=3.20

[Rik Mills]

rikmills added a reviewer: Plasma.

REPOSITORY
  R98 Breeze for Gtk

REVISION DETAIL
  https://phabricator.kde.org/D5128

To: rikmills, #plasma
Cc: plasma-devel, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, 
sebas, apol