display file icons before attempting to load Quick Look previews

As discussed in #1552, attempting to load Quick Look previews before giving up and showing the file's icon instead introduced a noticeable delay. I changed it to show the file's icon (if it exists) first. The obvious downsides are the extra work of loading two icons, and sending two notifications. But loading the file's icon is usually lightning quick. (Almost no one complained about it when it was done on the main thread.) And if there is no Quick Look preview, the second notification never happens.