Add a base class for a collection of ListItems.
Review Request #5599 — Created March 7, 2014 and submitted
This adds ListItemsCollection, which subclasses Backbone.Collection to
provide an event when fetching and to make constructor options available
as a variable on the collection.
Views can use the new "fetching" signal to show a loading indicator when
fetching a collection from the server.
Used this in a view. Accessed state on the options and responded to the