Remember the previous state of sidebar whenever a document is opened.

The sidebar is expanded every time a new document is opened. Even if the sidebar was collapsed during the last session, it is expanded in the next session. It is automatically expanded in every session because pps_metadata_get_boolean (priv->metadata, "sidebar_visibility", &show_sidebar); here returns TRUE even if show_sidebar was set to false during the last session.

This fixes #82 (closed) .