Fixes for special cases around root node in template

• If Snabbdom creates a new top-level element rather than patching DOMPatcher's existing element (for instance, when a key or tag change forces the creation of a new element), update DOMPatcher's internal element reference to point to it
• If Snabbdom recycles the existing top-level element that DOMPatcher newly created, it won't fire the insert hook since it didn't insert any new element into the DOM, so we run the hook from DOMPatcher directly if it's been set on the root node