Add missing file to VS solution.

Without this, we fail to find jpeg_cust_mem_init at link time on
Windows.
Improve XML parser.

Add a whitespace preserving mode, for future use with XHTML.

Also parse XHTML entities. This is not strictly according to spec,
but for properly formed XML documents it should not matter.
xps: Add fz_xml_is_tag to simplify tag matching.