#1 Simplify method for user customization

[John Merritt]

Generate files that are app_x.i, app_x_cb.c, etc. File
names with _x in them. This are files that the user
should modify. Generate these template files via
fd2python. For instance, app_x.i has the line %include
app.i which contains only the callbacks that are in the
*.fd file. All other callback routines, not part of
fdesign, are in app_x.i. Similiarly, for the C
wrappers. The user edits only app_x_cb.c and
app_x_cb.py and not the derived files app_cb.c and
app_cb.py.

The makefile should be adjusted appropriately.