Adwaita: Add focus to expander

We were not drawing a focus indication at all for
expanders. Bring it back. Also, make the color plane
focus snug.

Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/1899

gtk/theme/Adwaita/_common.scss

@@ -42,7 +42,9 @@ dnd {
  * Outlines *
  ********/
 
+expander-widget:focus:focus-visible,
 :focus-visible link,
+plane:focus-visible,
 label:focus-visible:not(.link),
 row:focus-visible,
 flowboxchild:focus-visible {
@@ -58,7 +60,6 @@ flowboxchild:focus-visible {
 }
 
 // Widgets that draw their focus indicator outset and not inset
-plane:focus-visible,
 scale:focus-visible > trough {
   outline-color: $focus_border_color;
   outline-style: solid;