[FIX] web: add style to focussed links in editable lists

In some forms (especially invoices), there are more than one action
"add a new line" on the invoice lines one2many. When using the keyboard
to access those links, some kind of style is required to identify which
link is currently focussed, else the focussed link is not visible.

addons/web/static/src/scss/list_view.scss

@@ -220,6 +220,13 @@
             width: auto!important; // Override the width computed in JS (needed for Chrome)
         }
     }
+
+    .o_field_x2many_list_row_add {
+        a:focus, a:active {
+            color: $link-hover-color;
+            outline: none;
+        }
+    }
 }
 
 // Buttons in ControlPanel