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Both GtkSourceView 4 and libgedit-gtksourceview 300 depend on GTK 3.
The “3” in “300” means that it targets GTK 3.
      For Vala code, use the GObject Introspection file, not the
      *.vapi, because the *.vapi file has been
      removed.
    
      The paths to *.lang and style scheme files have changed. For
      example,
      /usr/share/libgedit-gtksourceview-300/language-specs/
      is the usual location where *.lang files are found.
    
          GtkSourceMap has been removed.
        
          GtkSourceStyleSchemeChooserButton and
          GtkSourceStyleSchemeChooserWidget have been removed. See
          the Tepl library for a replacement.
        
          The macros and functions to check the library version have been
          removed: the gtk_source_check_version(),
          gtk_source_get_major_version(), etc functions; as well as
          all the macros such as GTK_SOURCE_CHECK_VERSION().
        
          The GtkSourceStyleSchemeManager:scheme-ids and
          GtkSourceStyleSchemeManager:search-path properties have
          been removed. Instead, you can connect to the
          GtkSourceStyleSchemeManager::changed
          signal to be notified.
        
          The gtk_source_style_scheme_manager_get_scheme_ids()
          function has been removed, use
          gtk_source_style_scheme_manager_get_schemes()
          instead.
        
All the GtkSourceStyleScheme properties have been removed. Use the get functions instead.
All the GtkSourceStyle properties have been removed. Use the gtk_source_style_get_data() function instead.