Stacked: add functionality to toggle stacked toasts

[Kennypt]

Do you want to request a feature or report a bug? feature

What is the current behavior? On mobile devices you need to touch outside of the toasts, but it would be useful to have a way to programatically toggle from collapsed to extended and vice-versa. For example when the view is full of toasts and I want to use the toast header click as a way to toggle collapse.

What is the expected behavior? One way could be to have a function that would toggle the collapsed state