Improve COPYRIGHT / LICENSE files

This follows the general approach of the Rust project.

Especially don't include bogus copyright information with non-existing
legal entities, or completely outdated copyright information but instead
explicitly state that the copyright of each contributor is retained and
the version control system can be used to get the full authorship
information.

Also rewrite the paragraph about the licenses of GNOME libraries to be
less specific and simply refer to their license information instead of
pointing to a non-existing LGPL file.

COPYRIGHT

@@ -1,68 +1,14 @@
-The Gtk-rs Project is copyright 2013-2016, The Gtk-rs Project Developers:
+The Gtk-rs Project is licensed under the MIT license, see the LICENSE file or
+<http://opensource.org/licenses/MIT>.
 
-Adam Crume <[email protected]>
-Adolfo Ochagavía <[email protected]>
-Andre Bogus <[email protected]>
-Anton Konjahin <[email protected]>
-Arne Dussin <[email protected]>
-Boden Garman <[email protected]>
-Brian Kropf <[email protected]>
-Bryant Mairs <[email protected]>
-Chris Greenaway <[email protected]>
-Chris Palmer <[email protected]>
-Corey Farwell <[email protected]>
-Daniel Zalevskiy <[email protected]>
-David Li <[email protected]>
-Edward Shaw <[email protected]>
-Edward Yang <[email protected]>
-Esption <[email protected]>
-Evgenii Pashkin <[email protected]>
-Geoffrey French <[email protected]>
-Gleb Kozyrev <[email protected]>
-Glenn Watson <[email protected]>
-Google Inc.
-Guillaume Gomez <[email protected]>
-Gulshan Singh <[email protected]>
-Jakob Gillich <[email protected]>
-James Shepherdson <[email protected]>
-Jeremy Letang <[email protected]>
-John Vrbanac <[email protected]>
-kennytm <[email protected]>
-Laurence Tratt <[email protected]>
-Lionel Flandrin <[email protected]>
-Lucas Werkmeister <[email protected]>
-Lukas Diekmann <[email protected]>
-Mathijs Henquet <[email protected]>
-Maxwell Koo <[email protected]>
-mitaa <[email protected]>
-Nick Herman <[email protected]>
-Nicolas Koch <[email protected]>
-Oliver Schneider <[email protected]>
-Ömer Sinan Ağacan <[email protected]>
-Ralph Giles <[email protected]>
-Paul Dennis <[email protected]>
-Paul Hendry <[email protected]>
-Philipp Brüschweiler <[email protected]>
-Raphael Nestler <[email protected]>
-Robertas <[email protected]>
-Romain Gauthier <[email protected]>
-S.J.R. van Schaik <[email protected]>
-Sebastian Schulze <[email protected]>
-Silvio Fricke <[email protected]>
-Simon Sapin <[email protected]>
-Steve Klabnik <[email protected]>
-Tobias Bales <[email protected]>
-trolleyman <[email protected]>
-Umur Gedik <[email protected]>
-UrKr <[email protected]>
-Vojtech Kral <[email protected]>
-Zach Oakes <[email protected]>
-Zach Ploskey <[email protected]>
+Copyrights in the Gtk-rs Project project are retained by their contributors.
+No copyright assignment is required to contribute to the Gtk-rs Project
+project.
 
-The Gtk-rs Project is licensed under the MIT license, see the LICENSE file
-or <http://opensource.org/licenses/MIT>.
+For full authorship information, see the version control history.
 
-This project provides interoperability with various GNOME libraries but doesn't
-distribute any parts of them. Distributing compiled libraries and executables
-that link to those libraries may be subject to terms of the GNU LGPL, see the
-LGPL file.
+This project provides interoperability with various GNOME libraries but
+doesn't distribute any parts of them. Distributing compiled libraries and
+executables that link to those libraries may be subject to terms of the GNU
+LGPL or other licenses. For more information check the license of each GNOME
+library.

LICENSE

@@ -1,7 +1,3 @@
-The MIT License (MIT)
-
-Copyright (c) 2013-2020, The Gtk-rs Project Developers.
-
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights