GTK fe: disabling next/back button during progressbar activity

[Attilio Fiandrotti <fiandro@tiscalinet.it>]

Hi

I've noticed that the next/back buttons are always sensitive during 
progressbar activity, so i think we should

-make them unsensitive when created in gtk_initialize
-make them unsensitive the bk/fw buttons at every call of gtk_progress_*
-let gtk_go() to set them (un)sesitive as needed, as it does right now.

what do you think?

attilio