In my opinion, we do not want to encourage use of this.

On the one hand, our libxml2 catalog support has never been thread-safe
and this seems to have caused problems in the past (looking at the
skins2 comments). On the other hand, the xtag plugin never implemented
catalogs, so XML parsers have to handle unknown XML tags gracefully in
any case.

Also, shipping DTDs with VLC is awkward and arguably a waste of space.

Also don't fail if the XML validator cannot be loaded.

getNextSibling was buggy (previous patch), and the only use doesn't seem right.

This patch fixes a regression brought by 88ba4526d03285a10e9abd59b9053c9d6927661
 that prevented arrows from being functional.

 - for some reason, this worked on Linux (?!?)
 - for some other reason, there doesn't seem to be 2 different modes for add_file

Close #4004

(manual forward port of 3a31af06dbabd4e21a68e46249fdf3209cd8060a)

As it works fine over http if mp4-files are done with mp4box -tight or similar way,
in most cases mp4-files aren't done like that and avformat works much better in those
cases (don't need to seek so much).

lua_osd: add some parameters to the osd.message function to allow the user to set duration and position.

While the configure checks were platform-agnostic, the implementation
was Windows-specific. Worse, this code now conflicts with our Unicode
filesystem layer, which uses the same function names. In practice, this
code was never used.

On Windows, wcstombs() converts UTF-16 to ANSI, not to UTF-8.

They may be inconsistent on broken multi-processor hardware. But the
system tick count is causing worse problems with power saving features.