Modified exhibit to properly handle asynchronous item loading.  

In particular, exhibit assumed that when a call to loadItems returned,
items were done loading and exhibit could display.  With introduction
of chunked load this was no longer true, so exhibit was rendering with
incomplete data.  Now added a callback to item loader, to allow
exhibit loading to continue.

At same time, introduced a "message" functionality to the busy
indicator, allowing exhibit to indicate what it is doing at any time
that the busy indicator is displayed.