Async TaggerProvider

Hi Gareth,

Tagger provider language services just create taggers for use with the document/view.  The taggers' GetTags methods are called synchronously by their callers since they are called during line layout.  You could probably update tagger data within them asynchronously and then notify via TagsChanged if there was an update and trigger a new GetTags call.  If you do that, you might want to have a snapshot approach where while loading new data async, you are still returning old data.  Then once the new data is fully loaded, flip over to returning that instead.