Sets order references from the move api

[solsson]

9 didn't have this, while addAfter had.

[solsson]

We rejected the idea to update 'previous' on existing elements, #18. Instead:

• a freshly prepared collection - i.e. as persisted - has no 'previous'
• the AuthoringCollection always maintains iteration order
• multiple adjacent adds are ordered accordingly at reconcile

This means that server-side reconcile uses 'previous' to identify added units (while also receiving modified units without 'previous'). When multiple 'previous' point to the same pre-existing unit they are persisted in order of appearance in the reconcile body.

[takesson]

Minor clarification: An added unit can also reference another added unit. It depends which order they were created. Both alternatives below should produce the same resulting document.

Original:

• Apelsin u1
• Dadel u2

Alternative 1:

• Apelsin u1
• Banan u12 prev = u1
• Citron u13 prev = u12
• Dadel u2

Alternative 2:

• Apelsin u1
• Banan u13 prev = u1
• Citron u12 prev = u1
• Dadel u2