Setup the main layout of the widget.

def setup_gui(self):
        """Setup the main layout of the widget."""
        layout = QGridLayout(self)
        layout.setContentsMargins(0, 0, 0, 0)
        layout.addWidget(self.canvas, 0, 1)
        layout.addLayout(self.setup_toolbar(), 0, 3, 2, 1)

        layout.setColumnStretch(0, 100)
        layout.setColumnStretch(2, 100)
        layout.setRowStretch(1, 100)