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_parser.cpp

@@ -27,11 +27,11 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 #include "variant_parser.h"
+
 #include "io/resource_loader.h"
 #include "os/keyboard.h"
 
 
-
 CharType VariantParser::StreamFile::get_char() {
 
 	return f->get_8();