[MCHANGES-412] Allow an action to carry an optional scope

**[Gary D. Gregory](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=garydgregory)** opened **[MCHANGES-412](https://issues.apache.org/jira/browse/MCHANGES-412?redirect=false)** and commented

In changes.xml, I would like an **action** to carry an optional {**}scope{**}.

This would help users understand what changes affect which part of their projects.

For example:

Fix some bug\
Bump dependency foo from 1 to 2\
Bump commons-parent foo from 1 to 2\
Documented this and that\

The use of the scope documents where the change applies, usually runtime, test, or build, but it could be string maybe.

This somewhat parallels the concept of a scope in a dependency so the value set could be the same as allowed for a dependency scope but that might be too restrictive.

Thoughts?

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No further details from [MCHANGES-412](https://issues.apache.org/jira/browse/MCHANGES-412?redirect=false)

Start with the changes.xml action examples and trace how action attributes are represented and consumed in the plugin. Define the optional scope behavior and its accepted values, then verify that scoped actions are accepted and their scope is visible to users without breaking actions that omit it.

Written by the indexing model from the issue text.