http-interface: - fixed large playlist loading failure

http-interface: - fixed large playlist loading failure

large playlists can take >10 seconds to load. since the xmlhttp object is
common within the page, multiple /status.xml requests would occurr (1 per sec)
before the playlist was loaded cancelling the /playlist.xml request.. i moved
the xmlhttp request object to a global variable. it's now checked to see if
there are any outstanding/active requests before submitting a new request. -
improved playlist display of large playlists.

Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>