Update child node with the data picker

[bkrmendy]

Consider the following element:

<div>Hello, {name}!<div>

We already show a cartouche in the navigator for {name}, but when a new variable is picked, instead of only replacing name, we remove all the child elements and replace them with the newly picked variable, leading to something like

<div>{updated.data.name}<div>

(notice that Hello, and ! are gone).

The expected outcome of the update is the following:

<div>Hello, {updated.data.name}!<div>