New non-stacking ToastDisplayer implementation

A rehash of PR #27.

This adds an interface for ToastDisplayer.

It deprecates Toaster, but enhances it by upgrading it to use the new interface (without breaking API).

It adds two implementations of ToastDisplayer:
- a regular one, like Toaster, but with the ability to cancel all preceding toasts
- a non-stacking one, which will only show the last toast

a proper factory you have one method & return on the type, that way you can add new types & keep the one method signature, this will do any way :+1: