Allow text annotation for message flows

[sombrek]

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

I'd like to add a text annotation to a message flow.

Describe the solution you'd like

Add a "Add text annotation" action to the context pad (like for sequence flows).

Describe alternatives you've considered

None.

Additional context

Theres an example of this in the spec.

[nikku]

Clear missing feature, moving to backlog for the moment.