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/string_db.cpp

@@ -27,8 +27,10 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 #include "string_db.h"
+
 #include "print_string.h"
 #include "os/os.h"
+
 StaticCString StaticCString::create(const char *p_ptr) {
 	StaticCString scs; scs.ptr=p_ptr; return scs;
 }