This commit fixes the append node command by caching and reusing created
shapes and bpmn elements.
This ensures we do not invalidate actions that build on these element
references.
Related to #6
This commit ensures we pipe invisible root elements (Process,
Collaboration) through our import infrastructure, too.
This way we we receive proper events for them.
Related to #6
This commit extracts the main import logic found in import/Importer into
the core/BpmnImporter module. By doing so we we are able to reuse it
during modeling.
Related to #6
Nico Rehwaldt