Feature: validator(s) to check ID uniqueness

[telic]

Several elements have identifying attributes that should have unique values within elements of their type. Add one or more validators to check that there are no duplicates.

Affected elements:

• ACT/@n
• SCENE/@n - but duplicates may exist between different acts
• DIV/@name
• L/@n - but duplicates may exist between different scenes
• MS/@n within tags within the same @t
• QLN/@n, TLN/@n, WLN/@n
  • should also consider MS of the same type (eg <QLN n="1"/> and <MS t="qln" n="1"/> overlap)
• PAGE/@n
• STANZA/@n when used outside of SCENE

Note that many (but not all) of these could be automatically fixed by the renumbering transformer.

[ubermichael]

I'd really prefer a way to express this in the schema, but I think the rules may be too complex.