indicate wether it is an "advanced" option or not. The goal with this is to
hide the user from all the strange options that very seldom needs to be
changed. added an extra argument to all the add_* macros to indicate wether
the item is advanced or not.
* src/libvlc.h: added an "show advanced options" config option default
disabled
* src/libvlc.c: the advanced options are not listed by vlc --help and
friends unless --advanced is used
* modules/gtk/preferences.c: the gtk/gnome preferences dialog don't show
advanced options unless advanced is set
all: set the advanced flag to VLC_FALSE or VLC_TRUE for all options, by
using some common sense. Please check and correct

note: All interfaces should still work like they used to, see the change in
modules/gtk/preferences.c to find out how to "hide" options.

second note: I think I've changed all the modules, but I could not test
everyone for compilation, expect some broken modules (the fix is to add ",
VLC_TRUE" as last argument to the add_* I forgot)