The hand-rolled stack combo we were using before
was looking at the visibility of the page itself
to show or hide items. Other stack switchers
expect us to use the GtkStackPage::visible
property for this.
The sidebar in the object page was using up more space than it deserves.
Replace it with a combo box in the subheader. Now that we have more space,
put the CSS node tree and the style properties in the same page again.
