Initialize the widget UI and return the UI.

def _init_ui(self) -> VBox:
        "Initialize the widget UI and return the UI."
        self._search_input = Text(placeholder="What images to search for?")
        self._count_input = BoundedIntText(placeholder="How many pics?", value=10, min=1, max=5000, step=1,
                                           layout=Layout(width='60px'))
        self._size_input = Dropdown(options= _img_sizes.keys(), value='>400*300', layout=Layout(width='120px'))
        self._download_button = Button(description="Search & Download", icon="download", layout=Layout(width='200px'))
        self._download_button.on_click(self.on_download_button_click)
        self._output = Output()
        self._controls_pane  = HBox([self._search_input, self._count_input, self._size_input, self._download_button],
                                    layout=Layout(width='auto', height='40px'))
        self._heading = ""
        self._download_complete_heading = "<h3>Download complete. Here are a few images</h3>"
        self._preview_header = widgets.HTML(self._heading, layout=Layout(height='60px'))
        self._img_pane = Box(layout=Layout(display='inline'))
        return VBox([self._controls_pane, self._preview_header, self._img_pane])