Make EditorWindow re-usable

These patches introduce 2 modifications that make
PyShell and EditorWindow easier to re-use by other
applications:

1 - We create the global variable use_subprocess in the
PyShell.py module rather thatn under __main__ in this
file. This allwos us to instanciate a PyShell without
using the script under __main__

2 - 3 new optional attributes have been added to the
EditorWindow constructor: top=None, height=None,
width=None. These arguments allow to specify the aprent
widget into which the EditorWindow has to be created as
well as the dimensions of the Editor (lines and
columns) use when the editor is created. The flist
option has been amde optional and the code has been
patch up accomodate flist being None.

We use PyShell to expose the Python interpreter running
out application PMV
(http://www.scripps.edu/~sanner/python/pmv) and the
EditorWindow as a python-syntax-aware editor in our
visual programing environment ViPEr
(http://www.scripps.edu/~sanner/python/viper).

Michel Sanner and Sophie Coon
sanner@scripps.edu