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.

src/SFont.c

@@ -4,6 +4,7 @@
     Copyright (C) 2003 Karl Bartel
 
     GrafX2 Modification
+    Copyright (c) 2011 Yves Rizoud
     Copyright (c) 2018 Thomas Bernard
 
     License: GPL or LGPL (at your choice)
@@ -29,6 +30,13 @@
     karlb@gmx.net
 */
 
+/* This file is based on SFont 2.03 was modified for GrafX2:
+  - SDL2 support
+  - Automatic space width detection
+  - Handling of missing lowercase characters
+  - Multiline support
+*/
+
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>