WIP: GtkTreeView

This is not finished but adds some very limited support for a "list box" which is realized in Gtk using a `GtkTreeView`

My "feasibility example" is

    using Gtk
    ls=GtkListStore((:gint,:gint))
    it=GtkTreeIter()
    append(ls,it)
    Gtk.set_value(ls,it,0,32)
    Gtk.set_value(ls,it,1,64)
    tv=GtkTreeView(ls)
    r=Gtk.GtkCellRenderer()
    c1=Gtk.GtkTreeViewColumn()
    Gtk.pack_start(c1,r)
    Gtk.add_attribute(c1,r,"text",0)
    c2=Gtk.GtkTreeViewColumn()
    Gtk.pack_start(c2,r)
    Gtk.add_attribute(c2,r,"text",1)
    Gtk.append_column(tv,c1)
    Gtk.append_column(tv,c2)
    win = GtkWindow(tv,"List View")

As this is quite verbose we will want to add a higher level api on top of that.