Bug 1105857 - [Messages] User doesn't receive any notification about incoming message if native modal dialog is shown. r=schung

Don't like that we should introduce both queuing and visibility flag, but both aren't supported by CustomDialog. For now I know only one case when we need this queuing - when user receives several class-0 messages in a row, we probably should keep every message. 

But what do you think? Maybe you have better idea in mind?