feat(interimElement): add onRemoving event

[blixt]

This event allows you to perform an action when e.g., a dialog starts to
close as opposed to when it has finished closing.

Fixes #3403.

Example use:

viewport.classList.add('blurred');
$mdDialog.show({
  controller: 'MyDialogController',
  clickOutsideToClose: true,
  onRemoving: () => viewport.classList.remove('blurred'), // <---
  template: myTemplate,
  parent: angular.element(document.body),
  targetEvent: event
});

[amiiit]

Thanks for this, I'd love to see this merged !!

[schwerlaut]

Ah, this is exactly what I've been looking for - great!