Enable resizing in session files.

[sirno]

This PR enables resizing windows in session files.

It is a bit of a hack because semantically I overload the window list in the session object.

Feel free to request changes.

[kovidgoyal]

I'd prefer it if the resize command was stored alongwith the window
launch spec in the windows list. You can have the windows list contain a
new class that stores the launch spec and the resize spec.

[sirno]

Yes that is indeed much clearer.

I adjusted the code to do this, however currently it will crash when resize_window is called before a window is launched. do you think there should be an error message for such a case?

[kovidgoyal]

Something provided before the first window should apply to the first window. That's how all the rest of the settings work, IIRC.