tests(import): add end event to simple fixture
chore(import): factor out import logic to own component

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
fix(import): handle invisible root elements

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
chore(tests): reactivate BPMN 2.0 rendering for tests

Related to #6
feat(lib/core): use directly linked data-model

Closes #91
chore(features/label-editing): directly depend on cmd

Related to bpmn-io/diagram-js#41
feat(features/bpmn-modeling): implement appendNode

Related to #6
chore(tests): increase karma timeout
feat(feature/bpmn-modeling): append task via context-pad

Related to #6, #40