Fix event handler method names checked for

Updating to Textual 0.27.0 broke this code, but it looks like they
made it way easier! It's now available statically on the class.

src/textual_paint/inspector.py

@@ -606,18 +606,7 @@ class NodeInfo(Container):
                     available_events.append(value)
         def message_info(message_class: Type[Message]) -> Text:
             """Return a description of a message class, listing any handlers."""
-            # A. Ideally Message would have a static method that returns the handler name.
-            # B. I tried constructing a message instance and getting the handler name from that,
-            #    with `message_class()._handler_name`, but:
-            #    1. this could have side effects,
-            #    2. this uses a private property, and
-            #    3. __init__ needs parameters, different for different message types.
-            #       (Constructing the base class and finagling it to use the subclass's namespace/name
-            #        might be possible, but seems like a lot of work for a fragile hack.)
-            # C. Duplicate the code from `Message.__init__`. It's not much code, since we can import camel_to_snake,
-            #    although I'm not sure the module is meant to be public, it's sort of just a helper.)
-            name = camel_to_snake(message_class.__name__)
-            handler_name = f"on_{message_class.namespace}_{name}" if message_class.namespace else f"on_{name}"
+            handler_name = message_class.handler_name
             handler_names = [handler_name, f"_{handler_name}"]
             # Find any listeners for this event
             # TODO: only look upwards if the event bubbles