Add the possibility to save the disk backend thumbnail and full view cache as subfolder in rawfiles folder.

My internal hard drive has very limited capacity and with the disk backend options turned on it fills up very quickly especially if you have a large database of raw files.

On the other hand, these options are very useful if you have a large number of directories with raw files and need to access them quickly and regularly to do post-processing of images when needed.

For example, I often get requests from my clients to additionally process a certain selection of photos for a certain purpose some time later, and if the disc backend option is not switched on, it can take a long time to (re)generate thumbnails and full view, especially for directories with a large number of raw files, which sometimes slows down the editing process enormously.

It is important to emphasize that the photos I produce on a daily basis are stored on various external hard drives. Accordingly, it would be beneficial to also have the thumbnails and full view previews for the purpose of quick access, in the sub-folders of the respective directories of raw files.

Describe the solution you'd like

1. Add in lighttable preferences under thumbnails additional option "save disk backend cache as subfolder in folder of raw files".
2. If this option is enabled, darktable creates "thumbnails" and "full view" subfolders in the raw files folder and saves thumbnails and full view previews in the respective folder.