Fix crashes on exit by properly exiting threads

The Python destructors (__del__) run much too late to be useful
to enforce proper shutdown order, i.e. stopping a QThread before
the application is torn down.

src/main/python/main/ayab/audio.py

@@ -85,8 +85,5 @@ def __init__(self, parent: GuiMain):
         self.__worker.moveToThread(self)
         self.start()
 
-    def __del__(self) -> None:
-        self.wait()
-
     def play(self, sound: str) -> None:
         self.__worker.play(sound, blocking=False)

src/main/python/main/ayab/ayab.py

@@ -20,11 +20,11 @@
 """Provides a graphical interface for users to operate AYAB."""
 
 from __future__ import annotations
-import sys
 import logging
 
 from PySide6.QtWidgets import QMainWindow
 from PySide6.QtCore import QCoreApplication
+from PySide6.QtGui import QCloseEvent
 
 from .main_gui import Ui_MainWindow
 from .gui_fsm import gui_fsm
@@ -100,6 +100,17 @@ def __init__(self, app_context: AppContext):
         self.fsm.set_properties(self)
         self.fsm.machine.start()
 
+    def closeEvent(self, event: QCloseEvent) -> None:
+        self.audio.quit()
+        self.audio.wait()
+
+        self.engine.cancel()
+
+        self.knit_thread.wait()
+        self.test_thread.wait()
+
+        super().closeEvent(event)
+
     def __activate_ui(self) -> None:
         self.ui.open_image_file_button.clicked.connect(self.scene.ayabimage.select_file)
         self.ui.filename_lineedit.returnPressed.connect(
@@ -135,7 +146,6 @@ def __quit(self) -> None:
         instance = QCoreApplication.instance()
         if instance is not None:
             instance.quit()
-        sys.exit()
 
     def start_knitting(self) -> None:
         """Start the knitting process."""

src/main/python/main/ayab/thread.py

@@ -32,9 +32,6 @@ def __init__(self, function: Callable[..., Any], *args: Any, **kwargs: Any):
         self.args = args
         self.kwargs = kwargs
 
-    def __del__(self) -> None:
-        self.wait()
-
     def run(self) -> None:
         try:
             self.function(*self.args, **self.kwargs)