Marks module in misleading place, so moved; ,maybe license issues

[GSR - FR <famrom infernal-iceberg com>]

Hi:

I see tabs were commited... and marks with it. But I do not see why
have marks under tabs. The mark module is perfectly valid contrib on
itself, limiting it to tabs does not sound fair, so I moved it to
util, and updated current tabs to match. Think about moving a handful
of windows from one workspace to another, for example.

I was even considering changing all terms from "mark" to "select", as
that is what I would think first when picking windows to do something
with a group, but I guess that can be misleading too, just in
different sense (people would probably expect any new command invoked
to work on selection), so I keep it as "marks".

I also updated the header of marks.jl, assuming the wiki page is
correct and this file is covered by GPLv3 or later. Please, everyone,
for next contribs, update licenses, docs and other inline comments.

GSR