Instead of having a configuration option, put the "normal" preview under the z key and preview with focus detection under ctrl-z.
preview with focusdectect is not mapped undr Z, it's mapped under ctrl-z
Remove: dt_iop_load_preset_interpolated_iso as not used.
New routine to check if a directory is empty.

Preparatory work for #9732.
Set id as a constant.
In dt_film_remove_empty() remove empty directory too.

When deleting or moving images the dt folders were properly deleted if
empty but the actual directory on disk was not removed. We now check for
empty folders and remove them.

For #9732.
Merge branch 'del-empty-folders'

* del-empty-folders:
  In dt_film_remove_empty() remove empty directory too.
  Set id as a constant.
  New routine to check if a directory is empty.
cut selection requests in chunk of 400 images

this fixes bug #9753
Merge pull request #390 from jkossen/preview_focusdetect_ctrl_z

[Post-1.4] Lighttable: Map preview under z, preview with focus detection under ctrl-z (Redmine feature request #9709)

Nice, tested and working fine. Thanks!