feat(bpmnrenderer): render X-OR marker only if 'isMarkerVisible' is set

close #69
fix(zoomscroll): fix using of incorrect offsetX/Y values for zoom on Gecko Browsers

close #83
refactor(zoomscroll): use layerX/Y instead more sophisticated solution.

As long as we use a container with relative positioning this should work.

close #83
feat(bpmntreewalker): improve 'boundary event' handling

- correct z-index of boundary event close #75
- also render 'boundary events' that have no flowNodeRef close #66
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