Sort out boxes and scrollables with GTK+ 3.2

Sort out boxes and scrollables with GTK+ 3.2

There is only GtkBox, neither GtkHBox nor GtkVBox.

Constructors and defaults have changed, we need
to set "orientation" for vertical boxes.

The panel takes care of its pages, and the view
of itself.

gtk_container_add on GtkBox is different to GtkVBox
so we must use gtk_box_pack_start.

GtkScrollable exists as a proper interface now.