Stacked: add functionality to toggle stacked toasts

**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

Start by locating the stacked-toast implementation and the public API used to control toast state. Review how mobile interaction currently changes the collapsed state, then define and test a programmatic toggle whose completion criterion is switching between collapsed and extended layouts.

Written by the indexing model from the issue text.