[Xmonad] dynamic workspaces, session restore

[Don Stewart]

valery.vv:
> Hi,
> 
> There are a couple ideas I would like to discuss.
> 
> 1) Fixed number of workspaces is too inflexible and somewhat ugly, IMO.
> 
>    What about creating workspaces by demand, instead? The same way
>    `ion' does to workspaces or screen to its "windows".

As pointed out, workspaces are created on demand (they're represented
with Nothing when empty), all you need to do is set the upper bound 
so we can bind keys.

-- Don