qt: save prefs on close more predictably

Ticket #379 reports crashes on quit due to `willSavePrefs` being called
when result and details dialogs are already freed. I can't reproduce the
crash, but it's still a bad idea to rely on the timing of
`aboutToQuit()` to launch this process.

This commits uses a more predictable place to emit `willSavePrefs` and
I'm pretty sure it will fix the crash at #379.

qt/directories_dialog.py

@@ -8,7 +8,7 @@ from PyQt5.QtCore import QRect, Qt
 from PyQt5.QtWidgets import (
     QWidget, QFileDialog, QHeaderView, QVBoxLayout, QHBoxLayout, QTreeView,
     QAbstractItemView, QSpacerItem, QSizePolicy, QPushButton, QMainWindow, QMenuBar, QMenu, QLabel,
-    QApplication, QComboBox
+    QComboBox
 )
 from PyQt5.QtGui import QPixmap, QIcon
 
@@ -232,7 +232,7 @@ class DirectoriesDialog(QMainWindow):
             if not self.app.confirm(title, msg):
                 event.ignore()
         if event.isAccepted():
-            QApplication.quit()
+            self.app.shutdown()
 
     #--- Events
     def addFolderTriggered(self):