Add `pagetree` macro

[osnard]

Example:

<ac:structured-macro ac:name="pagetree" ac:schema-version="1" ac:macro-id="...">
  <ac:parameter ac:name="expandCollapseAll">true</ac:parameter>
  <ac:parameter ac:name="root">
    <ac:link>
      <ri:page ri:content-title="@self"/>
    </ac:link>
  </ac:parameter>
  <ac:parameter ac:name="sort">natural</ac:parameter>
</ac:structured-macro>

Possible parameters: root, spaces, sort, excerpt, reverse, searchBox, expandCollapseAll, startDepth