[MASSEMBLY-939] - Aggregation Handlers support <updateOnly>

[roxspring]

https://issues.apache.org/jira/browse/MASSEMBLY-939

• Add test for AbstractLineAggregatingHandler behaviour
• AbstractLineAggregatingHandler uses latest source lastModified date of matched resources and supports <updateOnly>
• FileSelectors are always passed PlexusIoResource instances so that lastModified date is available
• ComponentsXmlArchiverFileFilter uses latest source lastModified date and supports <updateOnly>
• SimpleAggregatingDescriptorHandler uses latest source lastModified date and supports <updateOnly>

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