giscanner: Fix section matching for documentation
When writing documentation to the GIR files, GIR tries to match classes with their matching SECTION: comment in the source code. Some codebases use kebab-case or CamelCase for their section names, but GIR always expects them to be flatcase or the matching will fail.
This commit converts all section names to flatcase (by removing "-" and converting to lowercase) while they are being parsed, so that they are matched properly later on.
Fixes #350 (closed).