CSSUrl, remove quotes from the URL kept in .value

[hannibal218bc]

Previously, CSSUrl.value was stored verbatim, including any quotes around the actual value in url( ... ).
As .value is also used in getURL(), the resulting resolved URL included those quotes, which is wrong.

Example:

* {background: url("file.png");

With a base of http://www.example.org, the result from getURL() was http://www.example.org/"file.png".

This pull request modifies CSSUrl to remove balanced quotes from the input value before it is stored.
Admittedly, it has the side effect of removing the quotes also from the output of toString(), but they are optional and not needed anyways.