News in 3.15.3
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* Better word boundaries for word selection (double-click) and word movements
  (ctrl+arrow, shift+ctrl+arrow, ctrl+backspace, etc).
* Add style scheme chooser widgets: GtkSourceStyleSchemeChooser (interface),
  GtkSourceStyleSchemeChooserButton and GtkSourceStyleSchemeChooserWidget.
* Add gtk_source_view_indent_lines() and gtk_source_view_unindent_lines().
* Add the GtkSourceView::move-to-matching-bracket action signal.
* Add gtk_source_buffer_join_lines().
* Gutter: use current line background color for current line.
* Add syntax highlighting support for Apache Thrift.
* Various other improvements and bug fixes
* Translation updates