vdom.Helper: createDeltas(), main.mixin.DeltaUpdates: remove all nodes

This is an edge case, but an important one.

While playing with the table performance demo, I added 1k rows to a table body and removed them afterwards:

This makes no sense, in case there are literally no new children.

I added a new DeltaUpdates method: du_setTextContent(), which we can trigger with the setTextContent delta action property.

vdom.Helper: createDeltas() has now a new check for childNodes: in case the new vnode has no children and the old vnode has 1+, trigger the new delta action.

Result: