Allow lazy updates of header and body contents in Expander

Tristan's suggestion:

def getHeaderContent(self):
    content = self.headerFactory()
    if content == self.headerContent:
        return None
    content.setFragmentParent(self)
    self.headerContent = content
    return content

def updateRemoteHeaderContent(self):
    content = self.getHeaderContent()
    if content is not None:
        return self.callRemote(
            'setContentFromWidgetInfo', content, u'header')