license issue: mention original Expat/MIT licensing

[sunweaver]

This project has been forked from i3-hud-menu which is Expat licensed. The mate-hud project has been re-licensed under GPL-2+.

Thus, the file usrlib/mate-hud/mate-hud needs an additional reference to the Expat MIT license as some portions of the file may still be in their original (pre-fork) state.

The GPL-2+ license in cases like this only apply to the code changes committed after the fork.

Overall, re-licensing an Expat code project as GPL-2+ is possible and the effective license will then also be GPL-2+. However, code-documentation-wise the file contains code fragments that either Expat or GPL-2+ applies to.

In debian/copyright, I will list the mate-hud file like this:

Files: usr/lib/mate-hud/mate-hud
Copyright: 2016-2018, Martin Wimpress <code@ubuntu-mate.org>
          2015, JAMcNaughton
          2014, Rafaël Bocquet
License: GPL-2+ and Expat
Comment:
Copyright information obtained from upstream VCS log.
.
Project has originally been forked from i3-hud-menu
which is Expat-licensed.

In fact, mate-hud doesn't have a license header at all, so it would be good putting copyright years+holders, a copy of the GPL-2+ short form and a copy of the Expat license [1] to the file's header.

[1] https://github.com/jamcnaughton/hud-menu/blob/master/LICENSE