Use file watchers instead of document saved event

Thanks to file watchers, we already improved live preview for workflows that include a preprocessor and an associated watch task. For example, while editing SASS or LESS in live preview, if the user has appropriate tools in place to watch and compile files, we can reload the live preview browser to show changes. This is new behavior in sprint 36.

This pull request uses file watching in place of the `documentSaved` event from `DocumentManager` to trigger a browser reload. This allows external editors such as image editors, other text editors (ahem! :cold_sweat:) and background jobs to trigger live preview to reload, which should users very simple "live reload" behavior.