GtkLabel: don't eat too many button release events

We should only eat button release events when the label is
actually selectable, since the comment indicates that we
want to eat the release events belonging to press events
that triggered a selection. This fixes problems with actions
on parent widgets that are triggered by button release,
as seen in this bug:

https://bugzilla.gnome.org/show_bug.cgi?id=724541

gtk/gtklabel.c

@@ -4801,7 +4801,10 @@ gtk_label_button_release (GtkWidget      *widget,
   /* The goal here is to return TRUE iff we ate the
    * button press to start selecting.
    */
-  return TRUE;
+  if (info->selectable)
+    return TRUE;
+
+  return FALSE;
 }
 
 static void