Allow the user to select a directory for the autosaved image split to go into.

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