Better manage the composer:destroy event

 1. Use DOMNodeRemovedFromDocument instead of DOMNodeRemoved (catch all
    removals).
 2. Only add IE fallback interval checking code if MutationEvents are
    not supported.
 3. Use a MutationObserver instead of MutationEvents if supported as
    MutationEvents are deprecated and cause performance problems.
Merge branch 'better-destroy-event-handling'
Rebuild.