News in 3.23.1
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This version still uses GTK+ 3. A GtkSourceView 3.24 version is planned (still
depending on GTK+ 3), but the release date is not yet determined. If everything
goes well, GtkSourceView 3.90 (depending on GTK+ 3.90) will be released at the
same time as GTK+ 3.90 in March 2017.

* New class: GtkSourceSpaceDrawer with a matrix property to combine space types
  vs locations. The old white space drawing API has been deprecated.
* gtk_source_search_context_set_settings() has been deprecated, the "settings"
  property will become construct-only.
* GtkSourceCompletionItem: add a new API and deprecate the old constructors.
* GtkSourceGutter: add get_view() and get_window_type() public functions.
* Build system: do not hardcode the API version (currently 3.0) at as many
  places as possible (use a variable instead).
* Bug fixes
* Documentation improvements
* Translation updates