Change to gtksignal.h that would ease porting

[Darin Adler <darin bentspoon com>]

Since GCallback is defined as void (*)(void), but GtkSignalFunc was defined
as void (*)(...) in gtk 1.2.x, it would be nice if the deprecated macros in
gtksignal.h included a type cast. It does make things a bit less type safe,
on the one hand, but on the other hand it makes porting easier and people
shouldn't use these for new code.

Also, should GtkSignalFunc itself be deprecated, to be replaced by
GCallback?

And what about GtkArg? Is it replaced by GValue?

    -- Darin