Fixes #1104

* modules/gui/skins2/Modules.am: removed old skins2 theme.

   This doesn't fix totally the refresh problems on the skinned vout,
   but it improves the situation a lot.

  * skin.dtd : new BitmapFont element

* modules/codec/libmpeg2.c: modified last change to use a 4:3 aspect ratio when libmpeg2 gives us an invalid one.
  (if this happens, it is a bug in libmpeg2 anyway so we just try to fallback to the most usual value).

 (There is still the problem of timeout, and a lot of checks aren't done).

  * parser/builder.cpp: a font with ID "defaultfont" is loaded from
  the common font directory

  * all: added a "tooltipfont" element in the <Theme> tag (unused at the
      moment)

 * http.c: begin support of vlm interraction (not yet documented).

based to extension (others demuxers will still be probed if needed).