Alert hide() method

[bennypowers]

Proposal: add a hide() method to alert, which when called:
IF the alert is not dismissable, do nothing
ELSE perform the same steps as if the close button had been clicked, namely:

• fire a close event
• IFF the event is not defaultPrevented, remove the alert

This would help in implementing toast patterns where one alert must close before another can be presented.

See also #541