Adwaita: Bring fix focus in flowbox

Bring back the focus rectangle for flowbox children.
Its absence makes keynav in flowboxes really difficult.

gtk/theme/Adwaita/_common.scss

@@ -19,7 +19,8 @@ radiobutton:focus(visible),
 switch:focus(visible),
 scale:focus(visible),
 label:focus(visible),
-row:focus(visible) {
+row:focus(visible),
+flowboxchild:focus(visible) {
   // We use the outline properties to signal the focus properties
   // to the adwaita engine: using real CSS properties is faster,
   // and we don't use any outlines for now.