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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).

Only blu-ray subtitle are enabled.
If other formats are to be enabled, they must be carefully tested.

It will be used for blu-ray subtitles.