Schema: allow @component in more places

[jjrsylvestre]

Here are some places where @component is sometimes useful.

• docinfo/macros/@component
  Use case: sometimes a macro needs a different "workaround" definition for a specific output target.

• book/subtitle/@component
  Use case: full version versus abridged version, with no subtitle on the full version.

• idx/@component
  Use case: sometimes the index path/hierarchy for an entry is different when versions include/exclude material that contains related idx entries.