Specifies the type of function passed to
add_watch, which is called from the mainloop when a message is available on the bus.
The message passed to the function will be unreffed after execution of this function so it should not be freed in the function.
Note that this function is used as a GSourceFunc which means that returning
false will remove the GSource from the mainloop.
the Bus that sent the message
user data that has been given, when registering the handler