Request messages:
- operation and signal signatures
- arguments with and without names
- primitive argument values (no expressions)
- wildcard arguments

Reply messages:
- wildcard output parameter assignments
- unassigned output parameters with primitive values
- assigned output parameters with primitive values
- assigned operation return result, wildcard or with value
- unassigned operation return result

Signed-off-by: Christian W. Damus <give.a.damus@gmail.com>

Unit tests for some basic components:
- MessageParser for display of messages in the diagram
- utilities supporting the Xtext direct editor

As the Xtext direct editor is still being developed on the
Papyrus Sequence Diagram, isolate dependencies of
the direct editor and label parser on visual IDs in a
dedicated quarantine.

Implement qualification of references to lifeline
attributes assigned in reply messages.