x11-wm/wm{2,x}: add LICENSE (MIT) to Chris Cannam's window managers

Original sources did not include explicit license information, just
mentioned that a lot of code and structure were borrowed from David
Hogan's 9wm which AFAICT is MIT licensed.

Chris had confirmed with me via email that the MIT licence reflects
the general assumptions of the time and have added COPYING files to
the corresponding repositories:

https://hg.sr.ht/~cannam/wm2
https://hg.sr.ht/~cannam/wmx

PR: 279114, 279117