Validate XML ids and references

[oscarlevin]

It should be easy to add this checking. Possibly this is a function that you can run and get a report (like pretext-validate) or more appropriate for this tool, warnings shown as underlines in the editor.

[oscarlevin]

Note that we already maintain a list of xml:ids to suggest for xrefs. So just check whether the entered xref is on that list.

Should also be easy to check if that list has duplicates. Perhaps do something similar for labels.