Handles notifications from the data source's editing context,
looking for InvalidatedAllObjectsInStoreNotification and
ObjectsChangedInEditingContextNotification, refetching in
the former case and updating displayed objects in the latter.
WOImageButton renders a dynamically generated IMG tag or an INPUT tag, depending on whether the
element is inside a WOForm (in which case an INPUT of type IMAGE is generated) or not (in which
case the equivalent of a WOActiveImage)
src: A static URL for the image source.
data: A NSData object with the image content.