In order to use several different ctx from main().

No functional changes.

Regression from ad183c8f

It would have been used as NULL otherwise

and better handling of leaks

It need to be ported to the new player, playlist and tree.

The vout is now always present, there is no need to use the old playlist as a
proxy.

IMPORTANT: the video filter settings will now only change for the main vout. We
need to define a way to change vout settings for other windows.

The aout is always present, there is no need to use the old playlist as a
proxy.

Since the vout is now persistent, there is no need to store the variables on an
intermediate vlc_object_t (that was wrongly set to the interface during the
last UI merge).

IMPORTANT: the video filter settings will now only change for the main vout. We
need to define a way to change vout settings for other windows.

No real functional changes. Changes the callback and enum names to reflect the
real event: vout are now started and stopped. The same vout can be started and
stopped several time.

It must be replaced with legacy vout variables handling.

It must be replaced with legacy vout variables handling.