Add quit button

piket_client/gui.py

     QGridLayout,
     QInputDialog,
     QMainWindow,
+    QMessageBox,
     QPushButton,
     QSizePolicy,
     QToolBar,
         )
         self.toolbar.addAction(self.load_icon("undo.svg"), "Oeps")
 
+        self.toolbar.addAction(
+            self.load_icon("quit.svg"), "Afsluiten", self.confirm_quit
+        )
+
         self.toolbar.addWidget(self.create_spacer())
 
         # Right
 
             self.main_widget.init_ui()
 
+    def confirm_quit(self) -> None:
+        """ Ask for confirmation that the user wishes to quit, then do so. """
+        ok = QMessageBox.warning(
+            self,
+            "Wil je echt afsluiten?",
+            "Bevestig dat je wilt afsluiten.",
+            QMessageBox.Yes,
+            QMessageBox.Cancel,
+        )
+
+        if ok == QMessageBox.Yes:
+            LOG.warning("Shutdown by user.")
+            QApplication.instance().quit()
+
     @staticmethod
     def create_spacer() -> QWidget:
         """ Return an empty QWidget that automatically expands. """