Bug fixes and a couple of new events

Bug fixes (both reported by @capr 👍)

• #444 Fixed issue where attempting to save a empty content fixture (e.g Text element) caused the editor to crash.
• #443 Fixed issue where the _domRegion attribute against the editor became out of sync with fixtures on init, save and revert.

New features

• The editor now supports started and stopped events (to complement the start/stop events).