Add async before after visual marker as built in option in modeler

[StephenOTT]

One of the most frequent follow up questions on the camunda forums is where are the async save points in a bpmn image that is shared. Having the async markers built into the default modeler would save a lot of instruction giving to everyone when asking for additional info about where the async is or how to add a plugin.

[nikku]

Which visualization do you refer to?

[StephenOTT]

@nikku The typical vertical red line marker that is placed at the front or back edge of a activity to indicate its Aync Before and Async After configuration

[nikku]

Available as a plug-in.

I still see the value for having it in the core, though.