VID-1841 Make sure cat names use underscores

Either way works for me. However, to be crystal clear - in my opinion name "transformCatNames" suggest that method/function is actually changing something vs. returning something new. It seems like you could even make it a general purpose static method (not only for "categories" but for all sort of titles). I don't think it would be breaking any conception - as it's nothing new to PHP - think of methods like sort and ksort. But yeah - either way works for me.