close_window and new_window command closes #88, #90

Ok I've implemented `close_window` and `new_window` command both as `ApplicationCommand` and `WindowCommand`(as discussed [here](https://github.com/limetext/lime/issues/90)) it's a bit odd but appears that this is how sublime text works.
btw I am not sure about the `IsChecked()` method, how should this method work on commands like `new_window` or `close_window`.