mxml 4 is binary and source incompatible with 3, and there is no easy way to
stay compatible with both. Not great for a library. So replace with libxml2,
hopefully that is more stable. Also means we can get rid of all the mxml hacks
The change removes all direct calls to mxml from the modules that need an XML
parser (lastfm.c, pipe.c, rssscanner.c and httpd_rsp.c).
Even with the help of mxml, reading XML is hard, so a layer is added which
helps deal with stuff like whitespace and CDATA. This should make OwnTone more
resilient to any XML variations it might receive.
The changes fixes issue #1677.
