add exportConfiguration InfoObject to Node.save for predictable batchmode

Summary: As discussed with @rempt yesterday, I've modified Node.save
to accept this InfoOjbect just like Document.exportImage in order to
be able to save layers in batch mode with predictable output settings.

As it happens, I can't get Krita working with Python on my PC due to
incompatibilities between PyQt5 (v5.11.3) & Qt5 (v5.11.1) libraries. It
can't easily be solved as I couldn't find compatible packages, there's
no PyQt5 v5.11.1 release unfortunately and the change appears in v5.11.2

Patch by Razvan Radulescu (https://phabricator.kde.org/p/razcore.art/)

Reviewers: rempt, Krita
Reviewed By: rempt, Krita
Subscribers: rempt
Tags: Krita
Differential Revision: https://phabricator.kde.org/D17639