Include sample files for testing new *.config

[landroni]

It would be nice to have "sample" source files for some of the supported languages, to test all the language-specific stuff in the themes. The documentation suggests at least C++, Python, and XML, but I suspect other file types would be welcome too.
https://github.com/codebrainz/geany-themes/blob/master/ADDING-A-THEME.md#testing-the-conf-file

[landroni]

As a first batch of files, I would suggest to include some from Geany's source code. Some candidates:
https://github.com/geany/geany/blob/master/src/filetypes.c
https://github.com/geany/geany/blob/master/scripts/create_py_tags.py
https://github.com/geany/geany/blob/master/scripts/changelist.pl