Support for page sequences in target-counter function

This change makes the target-counter() function aware of page sequences. 

Until now when you have some introduction pages (title page, toc, etc) without page numbering and the actual numbering starting with 1 at the first content page, the target-counter function would always return the absolute page number of the target not the number relative to the page sequence containing that page.



Here is some example HTML:

```
<html>
<head>
    <style type="text/css" media="print">
        @page content {
            @bottom-right {
                content: counter(page) " of " counter(pages);
            }
        }
        a.toc-entry:after {
            content: " (" target-counter(attr(href), page) ")";
        }
        .content-block {
            page: content;
        }
    </style>
</head>
<body>
    <div>
        <h1>first page</h1>
    </div>

    <div style="page-break-before: always;">
        <h1>toc page</h1>
        <ul>
            <li><a class="toc-entry" href="#test1">link to toc entry 1</a></li>
            <li><a class="toc-entry" href="#test2">link to toc entry 2</a></li>
            <li><a class="toc-entry" href="#test3">link to toc entry 3</a></li>
            <li><a class="toc-entry" href="#test4">link to toc entry 4</a></li>
        </ul>
    </div>

    <div class="content-block" style="-fs-page-sequence: start; page-break-before: always;">
        <h1>content page</h1>
        <p id="test1">toc entry here</p>
        <p style="page-break-before: always;" id="test2">toc entry here</p>
    </div>

    <div class="content-block" style="-fs-page-sequence: start; page-break-before: always;">
        <h1>content page</h1>
        <p id="test3">toc entry here</p>
        <p style="page-break-before: always;" id="test4">toc entry here</p>
    </div>
</body>
</html>
```