[GTK3] Make CSD window decorations 18px instead of 16px
Summary: As mentioned by @ndavis in D23547, window buttons should be 18px instead of 16px. This revision changes the size of the window buttons accordingly.
Test Plan:
Before:
After:
Reviewers: Breeze, ngraham, ndavis
Reviewed By: Breeze, ngraham, ndavis
Subscribers: bcooksley, ndavis, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D23560