Some elements post custom messages on the bus on certain occasions, and in 0.10 this has become the primary means of communicating things to the application that don't need to be caught synchroneously, increasingly replacing GObject signals.

However, application developers have no way to find out whether an element will send certain custom messages on certain occasions or what they are called, at least not without checking the source code.

What we need is at least a simple mechanism for elements to register custom messages they post, preferably with an optional way to also register structure fields (name, field type, and description) for each of those custom messages.

That way gst-inspect can show this information to app developers.