Allow global overview and deletion of <bpmn:message> elements

[MaxTru]

Is your feature request related to a problem? Please describe.

Currently the user might create BPMN messages. These created messages will then exist on a global scope and can not be (I) viewed or (II) deleted (unless the user goes into the XML view).

Describe the solution you'd like

Following what we do in the Camunda Modeler (see camunda/camunda-modeler#1982), the user shall be able to view defined messages and potentially delete them.

Describe alternatives you've considered

See discussions in #272

Additional context

#272