ACTION-2264: Propose text for the merged textnodes case of insert and replace

All,

This action is a follow-up to ACTION-2261, discussed here:


https://lists.w3.org/Archives/Public/public-xformsusers/2019Sep/0007.html

We haven't actually decided which option to choose.

Two types of things can actually happen in instance data upon executing
`insert` or `replace` actions:

1. Zero, one or more nodes can be inserted into instance data.
2. Zero, one or two text nodes can be updated due to a merge.

An example of the latter case would be:

    <data>a<foo/>c</data>

where replacing `<foo/>` with the sequence `b<bar/>d` results in:

    <data>ab<bar/>dc</data>

Since we have only one insertion position, there can't be more than two
text nodes merges.

I would suggest adding the following event context information for
`xforms-insert` and `xforms-replace`:

    "updated-nodes (node-sequence)
    The instance data text nodes which have been updated by merging text
nodes, if any."

We can consider adding this to the `xforms-delete` event as well since node
merges can also happen in this case.

An example would be good as well.

-Erik