Introduce view.getUI

getUI is a stable interface to the view ui hash

An error is thrown when attempting to access undefined ui elements.
An error is thrown when attempting to access ui elements before the view has been rendered.
The jQuery collection matching the ui element selector is returned when the view is live.
An empty jQuery collection is returned after the view has been destroyed.

Fixes #2102
Yeah, that's covered by [this test case](https://github.com/duncanbeevers/backbone.marionette/blob/579c427005cbcd6f4ebe45ef046ebcfdedc7597f/test/unit/view.ui-bindings.spec.js#L30)