Libxfce4ui api review, round 2

[Nick Schermer]

Well for as long a nobody has complaints at this stage about the api,
we can start to port the core components to libxfce4ui. I also like to
purpose the switch to Gtkbuilder since the minimum gtk dep (2.14)
allows that*. I think we're better of doing that in branches until the
result is satisfying (git where are you?), if nobody has problems with
that I'll pick this up during the weekend.

Nick

* I do need to fix some issues in the gtkbuilder support in libxfce4ui
since gtk is not aware of the 4ui types (XfceTitledDialog), but there
is a quick code hack to fix that for the time being.