[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:
{F7291546}

After:
{F7291547}

Reviewers: #breeze, ngraham, ndavis

Reviewed By: #breeze, ngraham, ndavis

Subscribers: bcooksley, ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23560