3.21.3 (unstable)

* Add SearchContext ad SearchSettings.
  (Christoph Brill)
* Add StyleSchemeChooser, StyleSchemeChooserWidget and
  StyleSchemeChooserButton.
  (Christoph Brill)
* Buffer: Add implicit-trailing-newline property.
  (Christoph Brill)
* CompletionWords: Add activation property.
  (Christoph Brill)
* Language: Add get_style_fallback().
  (Christoph Brill)
* Style: Add style and scale-set properties.
  (Christoph Brill)
* View: Add get/set_background-pattern().
  (Christoph Brill)