[NEW PORT] x11/alttab: Task switcher for minimalistic window managers

[ultima]

Summary

X11 window switcher designed for minimalistic window managers (ratpoison,
xmonad and others) or standalone X11 session. It is lightweight and depends
only on basic X11 libs, conforming to the usage of lightweight WM.

WWW: https://github.com/sagb/alttab

PR\: 221425
Submitted by\: Dmitri Goutnik (maintainer)
Reviewed by\: lifanov (mentor), matthew (mentor)
Approved by\: lifanov (mentor), matthew (mentor)
Differential Revision\: https://reviews.freebsd.org/DXXXXX

Test Plan

portlint:
looks fine.

poudriere:
103i386
103amd64
110i386
110amd64
12i386
12amd64

[matthew]