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

I don't think so. It's a coincidence that they named their constant LENGTH_LONG.

`long` is used as an adjective in this case, not as a reference to any particular implementation.