impr(connection): Add rules to create connections

- sequence flow can only be modeled within the same diagram/pool
- message flow can only be modeled between pools
- associations can be modeled between all BaseElements

- Improvement: It is possible to easily connect objects using the context
  menue

- FIX: Fix import of Assosiations which has as source or target another
  Connection (ie. SequenceFlow, MessageFlow, etc.)

related to CAM-283, CAM-281, CAM-874