Add meta information to libvlc headers.

Add meta information to libvlc headers.

New doxygen command \libvlc_return_bool means that wrapper generators
should translate returning int to a boolean value.

For example, this is useful for Ruby, where every number, including 0,
is considered to be true.

For normal HTML/LaTeX documentation it's invisible.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>