tools(xml_tools): fixed `<div>` unwrapping + added `parse` instance: `<div>` unwrapping could fail when a text node was a sibling of the top element (could easily happen ith a `\n` line feed added by an editor). This is fixed by filtering on IElement with `elements()`. A `parse` instance has been added as it is not necessary to create a new `ElementParser` each time that we want to parse something.

version: "3.6"
services:

  sat:
    image: salutatoi/sat
    build: backend

  libervia:
    build: libervia
    depends_on:
      - sat