This is as per the work done in
http://lynnard.me/blog/2013/12/30/more-xmonad-goodies/, where the
functionality is explained in detail.
This also fixes#214. The original suggestion in that bug report is
not enough. Even if we export `gen` and `Uniq`, we would still have to
export the `Groups` type constructor. I thought it better to simply
allow the user to create a three-dimensional group instead.
This is a fix I have applied locally to make sure that when I use decorated layouts like tabbed in groups, XMonad does not leave bogus decoration windows lying around.
I think that the issue fixed is that the set of groups to send `Hide` to is determined by subtracting the extant groups from `l`, but `l` has already been put through `readapt` and so some groups may have been removed (if they are empty), so they don't get the Hide message.
The comparison should therefore be between `_l` and the new groups.
