Ensure we scroll correctly

First part of a new mono for audio filter 2. You can select the channel you want to hear all other channels are silenced.

TODO:
- stereo_to_mono downmix

* Cosmetic fixes in messages.

 * skins2/parser/interpreter.cpp: playtree.del() is now a synonym of
    playlist.del(), as it is already in use in some skins...
    Same thing for playtree.sort().

   by a previous commit

* Improve test output

* Redo dictionnary handling to remove recursion and bugs

  It works but it is not finished.
  - The changing verbosity does not work yet.
  - Too many messages are displayed ?

   - the format of the skins2-config variable has changed (not backwards-compatible)
   - the active layout and its size are now restored
   - the windows and layouts are now identified by their id, instead of their
     order in the XML file; this fixes problems of disappearing windows
     when editing a skin (commenting a Window for example)
   - it should now be impossible to have all windows hidden at the same time
     (but there is no check that the windows are on screen)

* modules/demux/mp4/mp4.c, src/input/decoder.c: support for cropping in quicktime files with H263 video.

* src/misc/messages.c, src/input/input.c: revert 16279 and use OBJECT_FLAGS_QUIET instead when preparsing.

* modules/access_output/shout.c: changed 'no bitrate information' warning to be used for both icecast and shoutcast.

* src/misc/messages.c: if OBJECT_FLAGS_NODBG is specified, don't output any messages at all. That avoids flooding the console with useless messages when preparsing.