Extend form-generator capabilities

[x] WONTDO: Support alternate displays for different subtypes (to support BibJSON). Have determined this will be covered by repeating groups added as needed.
[x] New render types: number, date, ...
[x] Implement "add repeating field" option to view edit (and entity view?)
[x] Support repeated field group (to support RecordView and BibJSON)
[x] Options for scoping enumerations (e.g. fields by recordtype); select entries with field matching value from containing form? The goal here is to prevent (say) the Bib* field entries swamping new view definition options.

gklyne / annalist