deliteful/Toaster widget

OK, I see your motiviation.   But I worry this mixed-type of `dismissibl`e will break declarative markup, because delite/Widget's code to interpret attributes doesn't know that `dismissible` (when specified) should be interpreted as a boolean.

Hmm.  Maybe you should make dismissible a boolean, default false, but show the button if dismissible == true *or* isExpirable() == true?    Or alternately make the default be true?   I'm not sure.