Make exo depend on libxfce4ui

[Nick Schermer]

Folks,

There is some (I agree, not a lot) of code in exo that is also in
libxfce4ui. It was always the plan to depend on libxfce4ui once it
replaced libxfcegui4.

This does not mean libexo depends on libxfce4ui, only the package (for
the preferred applications and desktop item editor dialogs).

For porting this means exo users also need to port to libxfce4ui if
they still use libxfcegui4. No something scary since gui4 is
deprecated, but still.

Anybody against this? Packages can also comment.

Nick