Style: Cleanups, added headers, renamed files

Made sure files in core/ and tools/ have a proper Godot license header
when written by us. Also renamed aabb.{cpp,h} and object_type_db.{cpp,h}
to rect3.{cpp,h} and class_db.{cpp,h} respectively.

Also added a proper header to core/io/base64.{c,h} after clarifying
the licensing with the original author (public domain).

core/variant.h

@@ -33,7 +33,7 @@
 	@author Juan Linietsky <reduzio@gmail.com>
 */
 
-#include "aabb.h"
+#include "rect3.h"
 #include "ustring.h"
 #include "vector3.h"
 #include "plane.h"