Quit dangerzone on macOS when window is closed

Closing windows on macOS would not actually close Dangerzone. Now that
it is a single-window program, it makes sense for it to close
immediately.

Fixes #271

CHANGELOG.md

@@ -12,6 +12,7 @@
 - Bug fix: Do not hardcode "docker" in help messages, now that Podman is also used ([issue #122](https://github.com/freedomofpress/dangerzone/issues/122))
 - Bug fix: Failed execution no longer produces an empty "safe" documents ([issue #214](https://github.com/freedomofpress/dangerzone/issues/214))
 - Bug fix: Malfunctioning "New window" logic was replaced with multi-doc support (([issue #204](https://github.com/freedomofpress/dangerzone/issues/204)))
+- Bug fix: (macOS) quit Dangerzone when main window is closed ([issue #271](https://github.com/freedomofpress/dangerzone/issues/271))
 
 
 ## Dangerzone 0.3.2

dangerzone/gui/main_window.py

@@ -100,10 +100,7 @@ class MainWindow(QtWidgets.QMainWindow):
             else:
                 e.accept()
 
-        if platform.system() != "Darwin":
-            # in MacOS applications only quit when the user
-            # explicitly closes them
-            self.dangerzone.app.quit()
+        self.dangerzone.app.quit()
 
 
 class InstallContainerThread(QtCore.QThread):