label: Fix mnemonic activation

The intention of the code is to find a focusable ancestor,
so it needs to look at the focusable property, not at
can-focus. This is a change from GTK 3, where can-focus
was the correct property to look at.

Fixes: #3965

gtk/gtklabel.c

@@ -1930,7 +1930,7 @@ gtk_label_mnemonic_activate (GtkWidget *widget,
 
   while (parent)
     {
-      if (gtk_widget_get_can_focus (parent) ||
+      if (gtk_widget_get_focusable (parent) ||
           (!group_cycling && gtk_widget_can_activate (parent)) ||
           GTK_IS_NOTEBOOK (gtk_widget_get_parent (parent)))
         return gtk_widget_mnemonic_activate (parent, group_cycling);