For automatically loaded files (directory loading), this filtering,
which requires additional disk access, was only done when the thumbnails
were displayed. But the thumbnail bar can be hidden, or the user can
request the display of the first or last image in the list before this
filtering has taken place.

Generally speaking, this filtering must therefore also be carried out
for any one-off image change operation.

This commit also fixes some old errors in the image list functions used
here, perhaps for the first time.

Related to !31, completes dafcfab7.