Make the default suggested action behavior to not show a preview.  Only override that for fixes/refactoring.

This is necessary so that the light-bulb will dismiss with long running
suggested actions.  Otherwise, it will stick on the screen for the duration
of 'Invoke()'.

Note: integration tests already do a 'wait' after invoking to make sure
the light bulbs have finished.  So they are unaffected by this.

Consume new async light bulb APIs. This makes the light bulb preview pane render asynchronously so that expensive previews don't block the UI thread.