### Problem Description
I use ubuntu linux. my system language is Chinese, but the dialog button labels is English by default. I found the buttonLabel option from the API to set the label of the save or open button, but the label of the cancel button is not supported.

### Proposed Solution
Support customizing cancel button label in Open and Save dialogs

shell/browser/ui/file_dialog_gtk.cc
public:
  FileChooserDialog(GtkFileChooserAction action, const DialogSettings& settings)
      : parent_(
            static_cast<electron::NativeWindowViews*>(settings.parent_window)),
        filters_(settings.filters) {
    const char* confirm_text = _("_OK");
    if (!settings.button_label.empty())
      confirm_text = settings.button_label.c_str();
    else if (action == GTK_FILE_CHOOSER_ACTION_SAVE)
      confirm_text = _("_Save");
    else if (action == GTK_FILE_CHOOSER_ACTION_OPEN)
      confirm_text = _("_Open");
    dialog_ = gtk_file_chooser_dialog_new(
        settings.title.c_str(), NULL, action, _("_Cancel"), GTK_RESPONSE_CANCEL,
        confirm_text, GTK_RESPONSE_ACCEPT, NULL);
    if (parent_) {
      parent_->SetEnabled(false);
      libgtkui::SetGtkTransientForAura(dialog_, parent_->GetNativeWindow());