app: properly grab focus when targetting text input of GimpSpinScale.

By just redirecting to parent signal handler, it was not properly
grabbing focus. In particular for a text entry with number input, we
want the text to be fully selected (because when we go for inputting
numbers, it is more often than not because we want to enter free numbers
very different from existing value).
So let's grab the entry focus, hence fully select current number
contents.

app/widgets/gimpspinscale.c

@@ -818,6 +818,10 @@ gimp_spin_scale_button_press (GtkWidget      *widget,
 
       return TRUE;
 
+    case TARGET_NUMBER:
+      gtk_widget_grab_focus (widget);
+      return TRUE;
+
     default:
       break;
     }