Utility method to bring overlays to the back.

[helenxywu]

Feature request to have a utility that can bring an overlay to the back (right now can only bring to the front).

This is useful in case we don't want a certain overlay to block other overlays (example: toast)

[valdrinkoshi]

Could you provide more details on the use case?
My understanding is that certain overlays (eg toasts) should always be on top, and if we open a modal dialog after a toast is opened, it should not block the toast. This can be achieved with alwaysOnTop http://jsbin.com/puqovenumi/2/edit?html,output