Add snackbar callbacks for onShow and onDismiss with documentation

[metalmatze]

The material design guidelines say that there shouldn't be more than one snackbar at a time.
To implement this guideline I created a flux store with all messages. These messages are in a queue.

Making it work properly, I need to have the onShow() and onDismiss() callbacks on the snackbar.

[hai-cea]

Thanks @metalmatze !

[rethink-rlinsalata]

Thanks for adding this, I was also trying to implement the "one snackbar at a time" guideline. However, I think to do it properly, the onDismiss() and onShow() callbacks need to occur after the CSS transitions finish.

The guideline says that the "When a second snackbar is triggered while the first is displayed, the first should start the contraction motion downwards before the second one animates upwards."
I was having trouble with my onDismiss being called too early, before the snackbar had actually closed. Thus, I was just updating the text without any noticeable actions.
Similarly, trying to check if there was an "active" snackbar by checking the .state.open was misleading, as the open state var changes immediately. Perhaps it needs another 'is hidden' variable? =/

Or, is there a better way to implement this "if (already open) close, then open" behavior?

[shaurya947]

@rethink-rlinsalata take a look at #1668