[Toast] Group notifications together

[xdaDaveShaw]

As per this REPL issue: fable-compiler/repl#46

@MangelMaxime suggested:

The feature I have in mind is to allow user attach a "group" to a Toast and say. I only want one Toast of this group displayed at a time.
If there is already a toast of the same group in the stack then it will be removed and the new one is added. This feature also needs to add transition/animation support so people can see the old toast being removed when the new one is added.

[xdaDaveShaw]

I'm happy to have a go at the F# changes for this - doesn't look too complicated... but I'd be stuck on the CSS changes needed to make the transition effects.

[MangelMaxime]

You can skip the CSS part as it is secondary for this feature.

When the feature is complete I can either work with you on the CSS side or implement it myself :)

[MangelMaxime]

I moved this issue to another repository, and further work will take place there.