Group all copyright statements in a single file.

This gives a much clearer overview of the licensing.

It also shows there are some problems:
- Some files are under GPLv3 only
- Some files have no known license at all.

CONTRIBUTING.md

@@ -6,4 +6,49 @@ Drive your browser there for a lot of useful information on GrafX2 development,
 internals, and roadmap.
 
 You can also join the #grafx2 channel on Freenode for more direct interaction
-with other developers.
+with other developers.
+
+GrafX2 is distributed under the GPL license, version 2 (with some files under
+less restrictive or compatible licenses, as indicated in the file headers).
+Please make sure the code you contribute to GrafX2 is suitable for distribution
+under such a license as mentionned in this certificate of origin:
+
+-------------------------------------------------------------------------------
+
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.