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

:x: :+1: Toast does hold a non-static ref, but in the StackingToastDisplayer (NonStacking is fine, as the list is cleared before a new toast is shown) the list grows and grows.