3.20.0:

* Builder chapter and example: Describe additional parameters to get_widget_derived()
  (Kjell Ahlstedt) Bug #134161
* CellrendererPopup example: Don't use deprecated methods.
  (Kjell Ahlstedt)
* Custom widget and ListBox examples: Update use of CSS files.
  (Kjell Ahlstedt)
* Custom widget example: Avoid some warnings.
  (Kjell Ahlstedt)
* ProgressBar example: Fix the type of a signal connection id data member.
  (Kjell Ahlstedt)
* Update the Internationalization chapter.
  (Kjell Ahlstedt) Bug #701908.
* gmmproc section: Add "String Parameter Processing" section.
  (Kjell Ahlstedt)
* C++11: Use nullptr instead of 0
  (Kjell Ahlstedt)