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.

While writing unit test, I found and fixed 2 more bugs with this RegExp:

1. The `[^\\]` phrase before each quote matches *any char except escape char (\)* so this would match a random char before each string. JavaScript doesn't support negative lookbehind, so this cannot all be done in a sinle RegExp, so first remove escaped quotes. Note that escaped quotes should only ever occur inside strings, but that is not enforced here -- should it be?

2. The `.*` is greedy, so if there are multiple strings in text, it matches a single string that includes everything between them. Changed to `.*?` for minimal match length.