AstroDataModel now has image_loader as field and provides a method to 
start loading images.  Also publishes image loaded messages, which 
include the source number and observation number of the image loaded.
This required adding this information to the loader classes so they can 
pass them in the callbacks.
ApplicationController now gets subscribed to MSG_IMG_LOADED events
from the model.  The loading dialog is shown on startup and then when 
there is an image loaded event the controller checks if it is the current 
image.  If it is, the dialog is hidden.

NOTE: there is currently some race condition being triggered in the 
underlying windowing code.  This causes the app to crash about half the
time.  Still debugging.