LinkField, PropField, FieldProxyConfig: add support for arrays of fields.
AmbientHacked: convert ambient_schema to a field_array.
Rendered: convert replacement_texture and joint_position to field arrays.
SpherePhysical: convert physics_radius and physics_offset to field arrays.

Lockable: remove lock and unlock methods, making set_locked public.

README: update requirements, including new build dependency on boost.