Adwaita: Avoid expander<>combobox interference

Putting a combobox in an expander was causing the combo arrow
to go sideways. Increase the specificity with which we address
the expander arrow to avoid that.

gtk/theme/Adwaita/_common.scss

@@ -3627,7 +3627,7 @@ row {
  * Expanders *
  *************/
 expander {
-  arrow {
+  title > arrow {
     min-width: 16px;
     min-height: 16px;
     -gtk-icon-source: -gtk-icontheme('pan-end-symbolic');