Setup the main layout of the widget.

def setup_gui(self):
        """Setup the main layout of the widget."""
        scrollarea = self.setup_scrollarea()
        up_btn, down_btn = self.setup_arrow_buttons()

        self.setFixedWidth(150)
        layout = QVBoxLayout(self)
        layout.setContentsMargins(0, 0, 0, 0)
        layout.setSpacing(0)
        layout.addWidget(up_btn)
        layout.addWidget(scrollarea)
        layout.addWidget(down_btn)