Draft: SVG2 flowed text

Switch to SVG2 flowed text for generating tutorial's SVG output using shape-inside="url(#id)":

Old method:

<rect id="id" />
<flowRoot>
  <flowRegion>
    <use xlink:href="#id" />
  </flowRegion>
  <flowDiv xml:space="preserve">...</flowDiv>
</flowRoot>

New method:

<rect id="id" />
<text shape-inside="url(#id})">...</text>

Why:

• Simpler.
• Fixes #15 (closed). There is no need for xml:space="preserve"
• Works with scour 0.37 (old method not, see #15 (closed)).
• Make possible to format XML to regular (pretty printed) format (this will be the next commit).

Partially fix #15 (closed) (left: ink:normalize-without-trimming).

WIP: We need scour 0.37 on the build server.