Use the model's mtime for validating thumbnails

Saves a syscall, generalizes fiv_thumbnail_lookup(),
wastes a tiny bit of memory per entry.

fiv-thumbnail.h

@@ -62,7 +62,8 @@ gboolean fiv_thumbnail_produce(GFile *target, FivThumbnailSize max_size,
 
 /// Retrieves a thumbnail of the most appropriate quality and resolution
 /// for the target file.
-cairo_surface_t *fiv_thumbnail_lookup(GFile *target, FivThumbnailSize size);
+cairo_surface_t *fiv_thumbnail_lookup(
+	char *uri, gint64 mtime_msec, FivThumbnailSize size);
 
 /// Invalidate the wide thumbnail cache. May write to standard streams.
 void fiv_thumbnail_invalidate(void);