put mono audio in both (left/right) channels. For some reason AOUT_CHAN_CENTER doesn't work. Somebody explain this to me, please.

* Does nothing useful at the moment (only useless device probing)
* The purpose would be to make an access-demux and an access sub-module reusing part of the code
* Access to a webcam or an analog card without an encoder (bt878) would help the development

* browsed through all code files starting with A to F and added non-blocking intf_UserFatal calls next to msg_err where I think that it is necessary.
I tried to select the most important msg_errs. If I missed your favourite, feel free to add it.
Feedback on the wording and the placement of the interaction calls is strongly welcomed.

For the records: I'm using some general topics for most of the dialogues to produce less work for the translators. The current ones go like this:
"Audio output failed", "Network interaction failed", "Streaming / Transcoding failed", "Audio filtering failed", "File reading failed", "Playback failure"

More to come.

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)