Parses text which is marked up with the Pango text markup language,
setting the label's text and attribute list based on the parse results.
If characters in text are preceded by an underscore, they are underline
indicating that they represent a keyboard accelerator called a mnemonic.
The mnemonic key can be used to activate another widget, chosen
automatically, or explicitly using set_mnemonic_widget().
GTK2.Label()->set_markup()