This fixes the issue that it was not possible to get any column back once all had been hidden

Some "no return statement in function returning non-void" warnings are
definitely not errors, and cannot be silenced without introducing dead
code, which is arguably just worse.

headers)

Added ability to move to previous item in the media list player playlist. Added playback modes to allow for automatic looping and repeating of playlist items. Fixed a race condition with Pause and Play functions by adding appropriate locks to these functions. Added automated testing for previous item and playback options.
Signed-off-by: Pierre d'Herbemont <pdherbemont@free.fr>

Partially revert 30f9eb49

* remove unneded p_sys variables (p_xml)
* fix memleaks.
* fix object leaks.

seems that noone is using it actually).