MEF based approach had race issue where if aggregator created after source, aggregator will miss events fired before aggregator created.

now user has a easy way to see just errors from build. the content should be exactly same as what output build window shows.

this change set also includes several misc changes such as adding support for error source and build tool.

now, error list will have column (which is off by default) that shows source of each errors in a
format "assembly name" for analyzers node. and "assembly name" [vsix name] for vsix