While arguably the image split should also automatically pick the location, because autosave implies things are all very auto, this makes the dialog a bit cumbersome: Either you are forced to untick autosave and type a file name for each cell from the image split or tick autosave and stuck with everything being saved into your already messy (don't pretend your home directory isn't a mess, dear commit reader) home directory.

This change asks for a directory once and then saves all the cells into it using the dialog settings.

Auditors: rempt