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/tile_set_editor_plugin.cpp

@@ -27,8 +27,10 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 #include "tile_set_editor_plugin.h"
+
 #include "scene/2d/sprite.h"
 #include "scene/2d/physics_body_2d.h"
+
 void TileSetEditor::edit(const Ref<TileSet>& p_tileset) {
 
 	tileset=p_tileset;