Currently PreviewView reacts to load-finished in two steps. First, its own load-finished handler is called, and then the load-finished handler from Embed sets the model.

Depending on the order in which handlers for the same signal are called is a bad idea. If the order got swapped, then the logic for showing the places and bookmarks controls will break because the check for multiple pages is only done when setting the model.

Let's consolidate things in one place.