signal/slot communication for the workspace model/view

[John W. Eaton]

I checked in another set of changes that alter the way Octave
communicates with the GUI:

http://hg.savannah.gnu.org/hgweb/octave/rev/0f143f68078d

This patch is for passing workspace info to the GUI using
signals/slots.  Updating the workspace view should be relatively fast
and reliable now but there is still room for improvement.  We are
still rebuilding the entire list every time Octave prompts for input.
Instead, we should be tracking whether the symbol table has changed
and only update if needed.  Further, we could cache the info attempt
to update only the portions of the data that have actually changed.

jwe