Move the default pos of the Paned handle to 400px from the left, i.e.
50% of the default width of the window. The previous position at 300px
from left meant the node treeview was too narrow & could easily result
in the (useful) State column not being visible in the case of many
apps. The properties pane doesn't need to be as big as it was anyway.
https://bugzilla.gnome.org/show_bug.cgi?id=788898
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.
Showing two lists in a paned was a bit awkward, and space was
getting too limited. Go back to showing just the node list at
first, and make the CSS properties available via a stack. At
the same time, add a right-click context menu to the node list
to make the name and class editing more easily available.
