Handle OS termination signals. (#425)

* Handle OS termination signals.

* Added comment about why a timer is required to handle OS signals.

qt/app.py

@@ -68,6 +68,8 @@ class DupeGuru(QObject):
         self.directories_dialog.show()
         self.model.load()
 
+        self.SIGTERM.connect(self.handleSIGTERM)
+
         # The timer scheme is because if the nag is not shown before the application is
         # completely initialized, the nag will be shown before the app shows up in the task bar
         # In some circumstances, the nag is hidden by other window, which may make the user think
@@ -166,6 +168,7 @@ class DupeGuru(QObject):
 
     #--- Signals
     willSavePrefs = pyqtSignal()
+    SIGTERM = pyqtSignal()
 
     #--- Events
     def finishedLaunching(self):
@@ -216,6 +219,9 @@ class DupeGuru(QObject):
             url = QUrl('https://www.hardcoded.net/dupeguru/help/en/')
         QDesktopServices.openUrl(url)
 
+    def handleSIGTERM(self):
+        self.shutdown()
+
     #--- model --> view
     def get_default(self, key):
         return self.prefs.get_value(key)