FEATURE: save lastly visited directory in the Reference Images docker

Now the docker saves/restores the lastly visited reference folder
over Krita restart.

** A question to painters **

Do you also need to restore the open reference images?

Please take into account that at the moment the list of reference
images is not per-painting, it is global, that is shared between all
open images in Krita.

CC:kimageshop@kde.org