'onclick is called on click with the tab offset as argument'

def __init__(self, offset, content, attr, onclick):

    p = urwid.Text(content, align='center')
    p = urwid.Padding(p, align='center', width=('relative', 100))
    p = urwid.AttrWrap(p, attr)
    urwid.WidgetWrap.__init__(self, p)
    self.offset = offset
    self.onclick = onclick
