Add a convenience method for reading "all text values of all child elements"

A relatively common case seems to be that of mapping structures like, say:

```

text
...
textN

```

Iterating over this structure is not super hard, but it would be nice to just have a method for "reading it all in", to produce map like:

```
{ "elem1" : "text", ... , "elemN" : "textN" }
```

and then let caller do whatever it needs to. Textual content should probably be simple collected with 'collectDescendantText', to work even if sub-elements were included.