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

:-1: whoa where's the conformity - in Toaster, the ctors are first.

@blundell I did this, and if I reformat with "rearrange entries" on, then it'll push the ctors to the top. What's your pref again?