Normal thumbnails generated even if shared thumbnails are available

On viewing a picture folder, shared thumbnails will be used if available (and if there are no normal thumbnails for the folder).

Though since thunar will create normal thumbnails, a f5 will not load the shared thumbnails any more, but favour the normal thumbnails (which is the right thing to do).

Here a small reproducer (for normal sized thumbnails): shared_thumbnail_test.tar.gz

The shared thumbnail of the wallpaper has a "S" painted on it .. so if you see that, you know the shared thumbnails are used.