Implement escaping for Strings

[rianmcguire]

We discovered a bug where unbalanced parentheses in a \Title string value would cause an invalid PDF to be generated.

Before:

$ echo '<title>abc(</title>' | weasyprint - test.pdf
$ qpdf --check test.pdf
checking test.pdf
PDF Version: 1.7
File is not encrypted
File is not linearized
WARNING: test.pdf (object 2 0, offset 92): EOF while reading token
WARNING: test.pdf (object 2 0, offset 858): unexpected EOF
WARNING: test.pdf (object 2 0, offset 858): parse error while reading object
WARNING: test.pdf (object 2 0, offset 858): expected endobj
WARNING: test.pdf (object 2 0, offset 858): EOF after endobj

After:

$ echo '<title>abc(</title>' | weasyprint - test.pdf
$ qpdf --check test.pdf
checking test.pdf
PDF Version: 1.7
File is not encrypted
File is not linearized
No syntax or stream encoding errors found; the file may still contain
errors that qpdf cannot detect

[liZe]

Thanks a lot for this PR!

We’ll probably use a regex instead of replace, for performance and readability, but we’ll definitely keep the idea. And thanks a lot for the extra tests!