Named entity recognition (NER) - modify span of existing annotation

**Is your feature request related to a problem? Please describe.**
We are using the NER functionality both to create annotations, but also to correct (imported) annotations. Sometimes the proposed span is wrong, and we want to manually correct this span.

**Describe the solution you'd like**
When hovering on the left or the right of an annotated span, a double arrow should appear that allows us to resize the span. The arrow looks like this:

**Describe alternatives you've considered**
We can remove and recreate the annotation, but this way we lose some information (which original concept was right with only a wrong span?).