New port: sysutils/twmn: Notification system for tiling window managers

There were some questions about the version. The source code has 1.2 in it,
but it hasn't been tagged on github. So I left it as g20171216.

PR: 224470
Submitted by: Dmitri Goutnik <dg@syrec.org>
Approved by: adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D13563