Adwaita: Fix hover in navigation sidebars

The placessidebar gets this from somewhere else, but
it wasn't working in the gtk-demo sidebar, so add it
explicitly. placessidebar specific styles should be
dropped from Adwaita as much as possible, but not
doing that here.

gtk/theme/Adwaita/_common.scss

@@ -3410,6 +3410,11 @@ placessidebar, .navigation-sidebar {
       }
     }
 
+    &:hover {
+      color: $fg_color;
+      background-color: $menu_selected_color;
+    }
+
     &:disabled { color: $insensitive_fg_color; }
 
     image.sidebar-icon {