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).

modules/gridmap/register_types.cpp

@@ -28,7 +28,7 @@
 /*************************************************************************/
 #include "register_types.h"
 #ifndef _3D_DISABLED
-#include "object_type_db.h"
+#include "class_db.h"
 #include "grid_map.h"
 #include "grid_map_editor_plugin.h"
 #endif