Notifications - support named notifications / prevent duplicates somehow?

[himdel]

Right now, there is no way to prevent multiple instances of the same notification at the same time, for example if you have an $http responseError interceptor that generates a session timeout message, multiple identical messages will be shown when multiple requests fail at the same time.

I'm not sure what the right approach to tackle that is, maybe we could get away with simply not adding a notification that deep equals to something already there.. Maybe something like having a concept of predefined notifications in Notifications might make more sense because then it should be at least easy to tell if it's already there or not...

What do you think?