cosmetics.
the Rules project item can neither be edited nor deleted.
set default order for render layer assignment rules to 1 in appleseed.studio.
added foundation::make_safe_filename().
derive filenames from aov names using foundation::make_safe_filename().
support variables in render layer names.

the following variables are supported:

    {entity-name}  the name of the entity
    {entity-path}  the full path to this entity, starting from the scene
Merge branch 'master' of github.com:appleseedhq/appleseed