chore(core): swap draw dependency

Make bpmn-js/draw depend on bpmn-js/core and not vice versa
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
wip(feature/context-pad): add basic implementation
wip(bpmnFactory)