Extend API with methods to edit resource text

[proycon]

The API needs some methods that allow editing a resource's text
(insertion,deletion,substitution). Editing text has consequences for
potentionally all annotations on it as they are referred to by offset. When
text is edited, offsets of other annotations need to adjust automatically. As
this is a fairly expensive operation, I want to implement the ability to
commit a batch of edits at once, so the computation can be done more efficiently.

When text changes and annotations are updated accordingly, the user should have the options of:

1. Just reusing existing resource and annotation IDs: annotations are forcibly edited. This is fine as long as the resource hasn't been published yet, otherwise discouraged.
2. Create new resources and new annotations with new IDs (support a version component in the ID): strictly adheres to the idea of annotations being immutable.
   • The old ones may be either retained or deleted
   • If retained, there may be an extra annotation linking the old ones to the new ones