display text. unless font_desc is provided, will use system's default font

def show_label(self, text, size = None, color = None, font_desc = None):
        """display text. unless font_desc is provided, will use system's default font"""
        font_desc = pango.FontDescription(font_desc or _font_desc)
        if color: self.set_color(color)
        if size: font_desc.set_absolute_size(size * pango.SCALE)
        self.show_layout(text, font_desc)