Add feature to link to a particular page or a page containing a given label

Add feature to link to a particular page or a page containing a given label

The main part of this patch was done by Dan Eble and posted to the
lilypond-devel mailinglist on December 27, 2010.

Like \with-url, this works only with the PDF backend.

-) Add markup functions \with-link (links to the page containing the
   given label) and \page-link (links to the given page number)
-) Internally, they are handled like the \with-url markup function,
   creating a pdfmark in the .ps file that creates the link in the
   PDF file.
-) Add links to the pages for all TOC entries.