correctly fixed arguments of the __new__ method

Fixed the arguments of the ```__new__``` method for the Dialog class changing it back to
 ```def __new__(cls, *args)```. Previously, these errors were being thrown when clicking either ```Save current session``` or ```Manage saved sessions...```:
```
Traceback (most recent call last):
  File "~/.local/share/gedit/plugins/sessionsaver/__init__.py", line 161, in on_save_session_action
    SaveSessionDialog(self.window, self, self.sessions).run()
TypeError: __new__() takes 1 positional argument but 4 were given
Traceback (most recent call last):
  File "~/.local/share/gedit/plugins/sessionsaver/__init__.py", line 164, in on_manage_sessions_action
    SessionManagerDialog(self, self.sessions).run()
TypeError: __new__() takes 1 positional argument but 3 were given
```
This was caused by ```__new__``` taking only one argument, which caused the above errors when ```SaveSessionDialog``` and ```SessionManagerDialog``` tried to create instances with more than one argument.