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

Review: https://phabricator.kde.org/D5128

This is a workaround to mitigate broken (huge wide) scrollbars in mozilla/Gecko based applications for GTK3 >= 3.20 in versions 52 onwards.
Ref: https://bugs.kde.org/show_bug.cgi?id=377008 and https://bugzilla.mozilla.org/show_bug.cgi?id=1343802
The mozilla bug is now 'fixed' and landing in nightlies v55, but may continue to affect earlier releases if not brought forward upstream.
Regardless of any other concern, applying this brings the appearance of newer breeze and breeze-gtk scrollars into closer accord.

BUG: 377008

Differential Revision: D5128