Rename `View#refind_significant_nodes` to `View#invalidate!`
Invalidate ViewComposer when parts are changed

Container/Partial now subclass View, which is now aware of what it has
been / is being composed with. This knowledge is passed to View objects
created during traversals.

When something changes (e.g. `remove` is called or `attrs` is set) the
View invalidates its related Composer if available.