games/libmanette: new port

Simple GObject game controller library

This is a new dependency to update games/gnome-games, particularly
games/quadrapassel to 40 (required) and www/webkit2-gtk3 to 2.32
(optional, enabled by default)

Reviewed by: nc
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D30628