Add support for open selects

[lognaturel]

As described at https://www.w3.org/TR/2003/REC-xforms-20031014/slice8.html#ui-selectMany:

Note that the datatype bound to this form control may include a non-enumerated value space, e.g., xsd:string, or a union of a enumeration and a non-enumerated datatype (called an open enumeration). In this case, control select may have attribute selection="open". The form control should then allow free data entry, as described in 8.1.2 The input Element. The form control may permit multiple values to be entered through free entry.

If the initial instance values match the storage value specified by one or more of the items, the all such matching items are selected. If the initial instance values do not match the storage value specified by one or more of the items, all such non-matching items are included as selected values, as if entered through free entry. Free entry text is handled the same as form control input 8.1.2 The input Element, possibly in multiplicity.

See forum thread at https://forum.opendatakit.org/t/allow-custom-choices-for-selects-that-get-saved-to-the-same-field/22788