nautilus-file: Don't show thumbnails for small zoom levels

When the zoom level is too small, thumbnails become useless, as they are
so small that they don't give any information.

The solution to this problem is to use mimetype icons when the zoom
level is too small

In order to achieve this, there was added a function that returns
true if the zoom is too small, nautilus_thumbnail_is_limited_by_zoom,
meaning that the corresponding mimetype icon should be used instead
of the thumbnail

https://bugzilla.gnome.org/show_bug.cgi?id=752805