preferences: Prevent ghost widgets glitch in Advanced by hiding all first

Hiding all Advanced preferences control widgets before deciding which to
unhide for the current selection avoids the situation where a previously
hidden widget is unhid and captures the background including a visible
widget about to be unhidden, resulting in the newly visible widget
restoring the previous widget's surface when drawn.

The bug this commit fixes only became possible when we gained a third
advanced preferences control widget/advanced preferences option type. It
is possible that the widget construction/placement/visibility toggling
orders are mismatched, but to be honest I'm not too interested in
messing with that further given this sensible workaround.