Always save last window session.

len(objreg.window_registry) can actually lag behind because single-shot QTimers
are used to remove the windows from the registry - but actually it doesn't even
matter if this is the last window or not. We just always save to
SessionManager._last_window_session, and it gets used in SessionManager.save.

Fixes #650.

qutebrowser/mainwindow/mainwindow.py

@@ -416,8 +416,7 @@ class MainWindow(QWidget):
                 e.ignore()
                 return
         e.accept()
-        if len(objreg.window_registry) == 1:
-            objreg.get('session-manager').save_last_window_session()
+        objreg.get('session-manager').save_last_window_session()
         self._save_geometry()
         log.destroy.debug("Closing window {}".format(self.win_id))
         self._tabbed_browser.shutdown()