meta: Make vlc_meta_t an opaque structure and move a bunch of static inline function to their private .c files.

This makes insertion/deletion faster, and saves a lot of code

This reverts commit 714d6155.

Conflicts:

	compat/tdestroy.c
	include/vlc_fixups.h

This patch fixes the following :
   - skins are now sorted alphabetically (Linux issue only)
   - the current skins is checked in list (instead of the first skins in list)

This reverts commit d98e43b1.

Sorry for the mess, missed it in the git rebase.

It applies hardware overlay constraints on a RECT.

This reverts commit e145f5a5bb6d61d67a8b9a6d1a4f1b33976c8ff6.

var_Change with VLC_VAR_SETVALUE showed a memory leak when dealing with a STRING variable.
The pf_dup function must not be called directy with a parameter passed by the caller.
otherwise, a reference is lost (memory leak)
Signed-off-by: Rémi Duraffort <ivoire@videolan.org>