BlockCanvas: Only can drop properties of the BlockCode's parent node

Godot allows drag any thing. However, if it is a property, block code
script only manipulates the properties of BlockCode node's parent node.
So, when drag a property, only allow drop the parent node's property.

Fixes: 89beea9 ("BlockCanvas: Implement drag & drop the node's property from Inspector")
https://phabricator.endlessm.com/T35649

addons/block_code/ui/block_canvas/block_canvas.gd

@@ -81,8 +81,10 @@ func _can_drop_data(at_position: Vector2, data: Variant) -> bool:
 	if typeof(data) != TYPE_DICTIONARY:
 		return false
 
-	# Allow dropping property block
+	# Allow dropping property block of the block code node's parent node
 	if data.get("type", "") == "obj_property":
+		if data["object"] != _context.parent_node:
+			return false
 		return true
 
 	var nodes: Array = data.get("nodes", [])