panel-plugin: replace XFCE_HVBOX with new GtkBox API.

The XFCE_HVBOX stuff is gone in the new version of the panel library
because its functionality has been superseded by GtkBox in gtk+-3.x.
First we drop the header include for libxfce4panel/xfce-hvbox.h that no
longer exists, and then we change xfce_hvbox_new() to gtk_box_new() and
xfce_hvbox_set_orientation() to gtk_orientable_set_orientation().