renamed components to components_old, to make room for some directory of the same name;

minor robustification;

added "isabelle components" tool;

tuned;

direct support for component forests via init_components; explicit ISABELLE_COMPONENTS_MISSING;

allow to provide external ISABELLE_IDENTIFIER for repository clone -- potentially relevant for isatest and mira; clarified spaces in file names -- ISABELLE_HOME is non-critical after abolishment of "make";