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/pool_allocator.cpp

@@ -27,12 +27,15 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 #include "pool_allocator.h"
+
 #include "error_macros.h"
 #include "core/os/os.h"
 #include "os/memory.h"
 #include "os/copymem.h"
 #include "print_string.h"
+
 #include <assert.h>
+
 #define COMPACT_CHUNK( m_entry , m_to_pos ) 			\
 do {								\
 	void *_dst=&((unsigned char*)pool)[m_to_pos];	\