libvlc: refactor sfilter enabling/disabling

Refactor sfilter enabling and disabling. One glitch stays present:
A user needs to call libvlc_{logo,marq,adjust}_int{ p_mi, libvlc_{logo,marq,adjust}_Enable, twice initially. Once before setting options to get module to load and second after setting options. After this Enabling/Disabling works as expected.
NOTE: when changing an option one MUST call
libvlc_{logo,marq,adjust}_int{ p_mi, libvlc_{logo,marq,adjust}_Enable, again.

The patch removes the dependency on the deprecated function vlc_object_find_name()
video_output/vout_subpictures.c: Fix filters appearing and disappearing when loaded for the first time.

The sys->filter_chain_update and sys->source_chain_update interacted with each other, because
the code was not carefull enough on when it is safe to overload 'sub-source' with the value of
'sub-filter'. The 'sub-filter' chain can also be a 'sub-source', but not the otherway around.