add a special case for generic application icons

It turns out that `[[NSWorkspace sharedWorkspace] iconForFileType:@"app"]` returns the generic document icon instead of the generic application icon.
show the file's icon before checking for a Quick Look preview

There can be a noticeable delay when trying to get a preview, only to find there isn't one and the file's icon should be used instead. So, show that first.

If there is no QL preview, calling `setIcon:` a second time with the file's icon is a no-op, so it should be fine.

fixes #1552
Merge pull request #1554 from skurfer/i1552

display file icons before attempting to load Quick Look previews