Filter the sidebar on keywords that can be provided
by the demos. We extract keywords from the doc comment
at the top of each demo source by looking for words that
look like class names. We also allow to specify keywords
explicitly.
We have a couple of Python 3.x scripts that parse C files, and since C
does not have any encoding, we need to force one ourselves, to avoid the
case when we're running the build in a non-UTF-8 locale.
https://bugzilla.gnome.org/show_bug.cgi?id=792497
