Libxfce4ui api review

[Nick Schermer]

I think I've already asked for this a while ago but at that time
nobody replied. So, it would be nice if developers can take a look,
I'll upload the API docs later today, but there is not a lot of code
so reading headers will do ;-).

One crucial missing piece is a session client, the reason is simple,
I'd like to have something close to the implementation gtk will use,
so I'm still waiting what they are going to do. /Me will pick this up
soon. Other than that most of the code is there.

So please review and post your problems so I can fix them! Then
hopefully we can send gui4 to the graveyard in Xfce 4.8.

Cheers,
Nick