Improve Taglist and Checklist to allow supplying of default values

[Skaiir]

Is your feature request related to a problem? Please describe

We decided to take default values from multi-selects out of the scope of our form-js improvements due to it requiring new components in the properties panel which is not trivial.

Describe the solution you'd like

We would need to sketch out some kind of a multi-select control that fits within the properties panel. That way we could define multiple values to be defaulted as part of the checklist, the taglist, and any future multi-select based control.

Describe alternatives you've considered

We could provide the values via comma separated values, at a much lower development cost. This could be an alternative, but at the same time we will most likely want a multi-select control for the properties panel one day or another.

Additional context

Taglist and checklist PRs:

#255
#262

[Skaiir]

Dependent on bpmn-io/bpmn-js-properties-panel#679