This is useful for plugins to be able to subclass the ScintillaObject
and use its full GObject features. The Overview plugin is one known
plugin that needs this.

Also update scintilla_changes.patch by running update-scintilla.sh
script with patch application commented-out and generating a reverse
diff using `git -R`, and manually remove `a/` and `b/` prefixes from
the resulting diff so it can be used with update-scintilla.sh script.