notebooks branch

[Owen Taylor]

I've pushed a branch now introducing Notebooks as a toplevel concept.
As I described a long time ago on:

http://www.reinteract.org/trac/wiki/NotebookDesign

a notebook is a grouping of related worksheets. To avoid a thousands
words, I've updated that page with a screenshot at the bottom.

To try it out, assuming you've already cloned the repository you would:

git pull # get latest changes from the repository
git checkout origin/notebooks # switch to the upstream notebooks branch
git checkout -b notebooks # Create a local tracking branch

And then run reinteract as normal. I would certainly appreciate
feedback about how this feels and works for people.

One thing that needs to be done before this is merged back to master
is fix up the handling of single worksheets - while I want people to
work in notebooks most of the time, if you double click on a
unattached .rws file, something reasonable needs to happen, which I
think is showing the worksheet in a window by itself. (.rws replaces
.pyw as the worksheet file extension, while I've used .rnb for the
index.rnb file found in every notebook. .rws seems have been used for
Borland resource files, but as affectionally as I remember Turbo C++,
probably not much of a concern at this point.)

- Owen