Install buttong into /usr/share/themes/Ambiance-Gentoo/metacity-1/

# Licensing
# https://bugs.gentoo.org/show_bug.cgi?id=499506#c6
"I (David Abbott) hereby license my altered buttons;

Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
http://creativecommons.org/licenses/by-sa/4.0/deed.en_US

Attribution:
The buttons were altered by me from the package light-themes from the
ubuntu-themes project. I changed the colors. The ubuntu-themes project was
not involved with these changes."
