Scale-independent overlays and tooltips

Hello. In my project that uses bpmn-js I overrided overlays module so it can render scale-independent overlays. They retain the same size regardless of current diagram zoom. Do you need this kind of functionality in diagram-js? I can make a pull request, but I am not good enough at writing unit tests.

Feel free to create a pull request. We can then look into that and make a decision.