Enable editing of multi-valued EAttribute annotations

* Add new classes
  * MultiValueAnnotationTextEditingSupport, an editing support that
    allows for textual editing of multi-value annotation.
  * MultiValueAnnotationTextEditingDialog, a dialog to that allows to
    edit multiple values
* Adapt
  * EditingSupportFactory to instantiate
    MultiValueAnnotationTextEditingSupport instead of throwning an
    exception about an unsupported case
  * EStructuralFeatureValueProviderBase to set values using the new
    method setAnnotationValueFromString(Collection<String>, T)

Issue-Ref: 3578
Issue-Url: https://af3-developer.fortiss.org/issues/3578



Signed-off-by: Simon Barner <barner@fortiss.org>