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

![](http://www.quickmeme.com/img/0c/0cd55ca159d7bbcec75627490fefbc2c032548978ede0385b18bf519f21ea2ff.jpg)