feat(connection point): drag and anchor reattach

Connection points can be moved
- if they represent bendpoints, bendpoint move is executed
- if they represent anchor points anchor reattachement is done

Known issues:
- we have to check for a reliable way to fire drag over/out events.
  currently drag over / out is to often firing
Merge branch 'messaging' into commandstack
refactor(commandstack) introduce command validation

its now possible to request the validation of a given command inside
a feature (or event callback), e.g. to give visual feedback on the
expected command execution result. the validation will always take place
before execution a command, so only valid and executed commands will be
pushed to the command stack