Add a get_class_name() method in the cel classes

I wish Godot had a method for this already

src/Autoload/ExtensionsAPI.gd

@@ -303,14 +303,5 @@ class ProjectAPI:
 		# then the old extension would have no idea how to identify the types they use
 		# E.g the extension may try to use a GroupCel as a PixelCel (if it doesn't know the difference)
 		# So it's encouraged to use this function to access cels
-		var project = get_current_project()
-		var cel = project.get_current_cel()
-		# Add cel types as we have more and more cels
-		if cel is PixelCel:
-			return {"cel": cel, "type": "PixelCel"}
-		elif cel is GroupCel:
-			return {"cel": cel, "type": "GroupCel"}
-		elif cel is Cel3D:
-			return {"cel": cel, "type": "Cel3D"}
-		else:
-			return {"cel": cel, "type": "BaseCel"}
+		var cel := get_current_project().get_current_cel()
+		return {"cel": cel, "type": cel.get_class_name()}

src/Classes/BaseCel.gd

@@ -73,3 +73,7 @@ func on_remove() -> void:
 
 func instantiate_cel_button() -> Node:
 	return null
+
+
+func get_class_name() -> String:
+	return "BaseCel"

src/Classes/Cel3D.gd

@@ -210,3 +210,7 @@ func load_image_data_from_pxo(file: File, project_size: Vector2) -> void:
 
 func instantiate_cel_button() -> Node:
 	return Global.pixel_cel_button_node.instance()
+
+
+func get_class_name() -> String:
+	return "Cel3D"

src/Classes/GroupCel.gd

@@ -19,3 +19,7 @@ func get_image() -> Image:
 
 func instantiate_cel_button() -> Node:
 	return Global.group_cel_button_node.instance()
+
+
+func get_class_name() -> String:
+	return "GroupCel"

src/Classes/PixelCel.gd

@@ -71,3 +71,7 @@ func load_image_data_from_pxo(file: File, project_size: Vector2) -> void:
 
 func instantiate_cel_button() -> Node:
 	return Global.pixel_cel_button_node.instance()
+
+
+func get_class_name() -> String:
+	return "PixelCel"