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).

tools/editor/plugins/resource_preloader_editor_plugin.cpp

@@ -27,13 +27,12 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 #include "resource_preloader_editor_plugin.h"
+
 #include "io/resource_loader.h"
 #include "globals.h"
 #include "tools/editor/editor_settings.h"
 
 
-
-
 void ResourcePreloaderEditor::_gui_input(InputEvent p_event) {