Roadmap for Modeler API 1.0.0

To do: Add some more issues (alpha target? beta target?)

1. Convert from annotations to attributes.
2. Use constructor promotion. (beta target)
3. Change names of class variables and methods that currently use "Eca".
4. Consistently spell modeler with one "l". (Can cspell help with this? I think it complains about e-mail with a hyphen; maybe we can get it to complain about modeller.)
5. Add documentation for updating to Modeler API. (README? Documentation on d.o?)
6. Decide what to do with the modeller_bpmn module. (Move to modeler_api module or bpmn_io module or something else? Or keep it in the eca package?)

This is looking pretty good, only a couple issues left open.