Deprecate createMessage function

[howawong]

createMessage function is too similar to addMessage function. It is better to deprecate createMessage.

[carmenlau]

Agree that addMessage is more flexible for sending a message, but for me I think createMessage looks like to shorthand for user to sending message that they don't have to create the message object. So we could keep both? :P

It is not urgent, so I just remove it from the milestone first

[rickmak]

According to design principle

Obvious means that API references should clearly reveal how to do something. Avoid redundancy for the same use cases. Make the API obvious how to do certain things. Avoid acronyms. It will be easier to learn and easier to remember for users.

We should provide one obvious way to do one thing.