Fix style sheet parsing

This is for #9875 .

When minimizing style sheets for parsing, limit string removal to content property.

Added unit tests to CSSUtils and Quick View.

@dangoor The problem with HAML files is that *everything* is in quotes (and no need to balance delimiters). Maybe the best fix is to limit the files scanned by Quick View?

If so, which languages or modes? Should this have a preference?