Add getIdentities function

I implemented your suggestions from https://github.com/protz/GMail-Conversation-View/pull/866.

Should getDefaultIdentity() rather return only the nsIMsgIdentity-object instead of the struct? isDefault would always be true!
I don't have a strong opinion for whatever this function should return. It may be useful to make it return the same kind of objects as `getIdentities`, since the client code may have a function that takes that type of objects... so I think it's good as it is.