Book navigation panel should be optional

It is impossible to permanently remove the navigation panel from the UI. That's because there is a setter called from the library sheet and this setter is adding the panel in the wiki preferences. So, whenever a page using the library sheet is loaded, the panel is added to the UI.

This way of adding elements in the UI is not desired in general, as it limits the decisions that the user can make in this regard.

My proposal is to remove this setter and leave the decision of adding the panel in the UI in the hand of the user.