New port: x11-themes/materia-gtk-theme

Materia (formerly Flat-Plat) is a Material Design theme for GNOME/GTK+
based desktop environments. It supports GTK+ 3, GTK+ 2, GNOME Shell,
Budgie, Cinnamon, MATE, Unity, Xfce, LightDM, GDM, Chrome theme, etc.

PR: 235914
Submitted by: Hiroki Tagato <tagattie@yandex.com>
Reviewed by: tcberner (mentor)
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D19338