Download button click handler: validate search term and download images.

def on_download_button_click(self, btn) -> None:
        "Download button click handler: validate search term and download images."
        term = self._search_input.value
        limit = int(self._count_input.value)
        size = self._size_input.value
        if not self.validate_search_input(): return
        self.clear_imgs()
        downloaded_images = download_google_images(self._path, term, n_images=limit, size=size)
        self.display_images_widgets(downloaded_images[:min(limit, 12)])
        self._preview_header.value = self._download_complete_heading
        self.render()