window: Stop using `decoration-button-layout` unless it is set
Hello,
This commit is greatly based on commit 301e1a4d:
commit 301e1a4d
Author: Timm Bäder mail@baedert.org
Date: Sat Oct 15 21:56:09 2016 +0200window: Remove decoration-button-layout style property
GtkSettings has a gtk-decoration-layout property for this.
For backward compatibility purposes, however,
the decoration-button-layout
style property is not
completely removed: it is still taken into account
if a theme explicitly sets this property. Otherwise,
the gtk-decoration-layout
GtkSettings property is
used.
Closes #5499.
Thank you!
snip