[Gtk-sharp-list] SignalHelpers are now internal

[Mike Kestner]

I managed to squeeze a couple hours of hacking in tonight and committed
the internalization of the signal helpers.  

I wasn't able to finish the Callback helpers though.  There are a half
dozen methods in the Gnome namespace that use Gtk callback delegates and
need access to a helper.  My plan is to update the code generator to
duplicate the callback helpers where necessary if a method param
utilizes a Callback in an external assembly.

It should be safe to remove the 109 *Sharp.*Signal classes from the
documentation now since they are no longer exposed.

-- 
Mike Kestner <mkestner@speakeasy.net>