Label Parsers for Messages

## Summary

The lightweight sequence diagram needs GMF *Label Parsers* for presentation and direct-editing of message names in the diagram.

## Initial Requirements

### R1. Message numbering
The user must have the option to
* **R1.1** show or hide message numbering in the diagram
* **R1.2** when showing message numbers, use either
* **R1.2.1** simple numbers: 1, 2, 3, ...
* **R1.2.2** hierarchical numbers: 1, 1.1, 1.2, 1.2.1, 1.3, 2, ...

This numbering is apart from any other content of a message label, as described in following sections. Moreover, it is not accessible in the direct editor; it is strictly computed from the sequencing of messages in the interaction model. Message numbers are presented before (to the left of in left-to-right locales) the message label as specified by UML.

Hierarchical numbering conforms to the *sequence expression* specification for *Communication Diagrams* in §17.9.1.3 of the [UML 2.5.1 specification](https://www.omg.org/spec/UML/2.5.1/). This includes the `a`, `b`, etc. names for concurrent messages in a parallel combined fragment.

### R2. Message Signatures
If the message references a behavioral feature as its *signature*, then a textual representation of that feature should be shown as the label. This will typically be an `Operation` of the receiving class (actual behavioral feature) or a `Signal` (implied behavioral feature) for which the receiving class has a `Reception` (that being the actual feature). Or, in the case of a component *port* receiving the message, an `Operation` of some provided `Interface`.

*Note that, per §17.4.3.1 of the [UML 2.5.1 specification](https://www.omg.org/spec/UML/2.5.1/), a message that has a signature must have the same name as that signature. Accordingly, the name will be fixed and not editable for any message that has a signature.*

### R3. Direct Editing
Support for direct editing of the message label must conform to the label syntax prescribed in §17.4.4.1 of the [UML 2.5.1 specification](https://www.omg.org/spec/UML/2.5.1/). Three input schemes are offered for the user to choose from (in the diagram preferences):
* **R3.1** structured label: as per the specification cited above, with syntax highlighting and content-assist for selection of *signature* names, operation parameter names, and signal attribute names from the contextual model based on the type of the `Lifeline`'s represented `ConnectableElement`. Conformance to the UML standard syntax is enforced by the in-line editor
* **R3.2** free-form label: the user can type any text. No syntax highlighting, content assist, or validation is provided
* **R3.3** hybrid: the user can type any text, with content-assist to select the name of the *signature* element. No validation is provided, nor syntax highlighting

As indicated in the description of **R1**, above,
* **R3.4** the sequence number (if any) is not presented in the *Direct Editor* for editing by the user