by having two booleans "export_backward" and "export_forward" in the
api instead of just an "export" one that would destroy the "imported
from" state. This change fixes the state of the "Overwrite" menu item,
so it stays visible until the file got either saved or exported to
another filename.

This also reverts commit a4beeecf, so
Ctrl-S is always invokable even if invisible.