Replace ExoTreeView with GtkTreeView or XfceTreeView

ExoTreeView will be deprecated during cycle 4.21, and exo 4.22 will probably be the last exo release before archiving. XfceTreeView (from libxfce4ui) replaces ExoTreeView, but is not derivable as ExoTreeView was. Xfburn is the only component that used to derive one of its widgets from ExoTreeView, so I think it's up to it to deal with the new situation, either by deriving from GtkTreeView instead (I made this change locally, as I don't have the impression that the additional ExoTreeView features were really being used, and it doesn't seem to be a problem), or by packaging XfceTreeView in a GtkBox.