Preferences, singletons

Create a Preference dialog for fonts and audio output devices

Turn Preferences and SoundOff into singletons.  No matter how many times
you request a new one, the same instance is returned.

Stop using singals on the parent() to access other instances, such as
sound and Preferences.

main.py

@@ -44,6 +44,7 @@ from PyQt6.QtWidgets import (
 )
 
 from lib import *
+from lib.preferences import Preferences
 from ui.MainWindow import Ui_MainWindow
 
 
@@ -60,8 +61,6 @@ def query_error(query: QSqlQuery) -> None:
 
 
 class MainWindow(QMainWindow, Ui_MainWindow):
-    playAlert = pyqtSignal()
-    playSound = pyqtSignal(str)
 
     def __init__(self) -> None:
         super(MainWindow, self).__init__()
@@ -73,8 +72,7 @@ class MainWindow(QMainWindow, Ui_MainWindow):
             os.path.join(os.path.dirname(__file__), "ui/resources.rcc"), "/"
         ):
             raise Exception("Unable to register resources.rcc")
-        self.soundOff = SoundOff()
-
+        
         model = QSqlQueryModel()
         query = QSqlQuery("SELECT * FROM people ORDER BY name")
         model.setQuery(query)
@@ -86,23 +84,28 @@ class MainWindow(QMainWindow, Ui_MainWindow):
         #
         # Action Connections
         #
-        self.actionQuit.triggered.connect(self.close)  # Y
-        self.actionAddBook.triggered.connect(self.addBook)  # Y
-        self.actionEditBook.triggered.connect(self.editBook)  # Y
-        self.actionRead.triggered.connect(self.readBook)  # Y
+        self.actionQuit.triggered.connect(self.close)
+        self.actionAddBook.triggered.connect(self.addBook)
+        self.actionEditBook.triggered.connect(self.editBook)
+        self.actionRead.triggered.connect(self.readBook)
         self.actionRead.enabledChanged.connect(self.readBtn.setEnabled)
-        self.actionAddPerson.triggered.connect(self.addPerson)  # Y
-        self.actionEditPerson.triggered.connect(self.editPerson)  # Y
+        self.actionAddPerson.triggered.connect(self.addPerson)
+        self.actionEditPerson.triggered.connect(self.editPerson)
         self.actionEditPerson.enabledChanged.connect(
             self.editBtn.setEnabled
-        )  # Y
-        self.peopleView.doubleClicked.connect(self.editPerson)  # Y
-        self.peopleView.clicked.connect(self.selectedPerson)  # Y
-        self.playAlert.connect(self.soundOff.alert)
-        self.playSound.connect(self.soundOff.playSound)
+        )
+        self.actionPreferences.triggered.connect(self.editPreferences)
+        self.peopleView.doubleClicked.connect(self.editPerson)
+        self.peopleView.clicked.connect(self.selectedPerson)
         self.show()
         return
 
+    @pyqtSlot()
+    def editPreferences(self):
+        dlg = Preferences()
+        dlg.exec()
+        return
+
     @pyqtSlot(QModelIndex)
     def selectedPerson(self, index: QModelIndex) -> None:
         person_id = index.siblingAtColumn(0).data()