Improved simple data migration

[ebruchez]

With #1523 (released in Orbeon Forms 2018.2), we implemented simple data migration.. The feature supports only:

• add and remove controls
• add and remove grids and sections

But we have some inconsistent behavior related to moving fields:

• Even without simple data migration, moving a field within a grid produces a data format that remains compatible (existing data loads as expected).
• But moving a field between two grids, since the 2019.1 data format, is not compatible. Neither does moving a field between two sections. In both these cases, simple data migration kicks in and the field will still show, but existing data for that field is lost.

We should improve simple data migration to support moving fields.

For every XML element, identified by name, that is removed from a place and added to another, not only, as is done now, delete/add the element, but also copy its content, including:

• text content
• nested <_> (multiple attachments)
• attributes values, if compatible (attachments, @label, others?)

Possibly unsupported: moving a field from a non-repeated container to a repeated container and vice-versa.

This would allow us to tell form authors that simply moving a form control in a form, no matter where (except repeated content) doesn't require publishing a new form version.

[ebruchez]

The way simple data migration works is that we gather Delete and Insert operations. It should be easy to go through that list of operations and match Deletes and Inserts that have the same element name. If that's really all there is to it, then it's a super simple implementation.

• [x] try simple implementation
• [x] move outside of repeat
• [x] also handle move within the same repeat iteration
• [ ] check impact on HolesOnly and mergeXmlFromBindSchema (used for XML import only)
• [x] unit tests
• [ ] doc