CasEditor: Allow creating new annotations "on top" of existing annotations without manually defining the span

It would be great to allow adding an annotation "on top" of an existing annotation without the need to manually define the span.

E.g. In order to add an arbitrary annotation on sentences using existing, let's say, "Sentence" annotations, it would be great to be able to do the following:
1. open the "Sentence" type in the FeatureStructure
2. iterate over all annotations of the "Sentence" type
3. add arbitrary annotation (e.g. using "Annotate" or "Quick Annotate") over the same span as the selected sentence annotation

This makes the CAS Editor a good choice for "markable"-base annotations, comparable to http://mmax2.net/ with all advantages of UIMA.