Determine `root` of a Tree-module from Jinja is... ugly

[phorward]

In a current ViUR project, this has been done to read entries from a menu organized by a Tree-module:

{%- set root = getSkel("menu", "rep_module_repo", skel_args=["node"]) %}
{%- set menu = getList("menu", skel_args=["node"], limit=50, parententry=root["key"]) %}

This is really somewhat ugly. Why can't it be something like

{%- set menu = getList("menu", skel_args=["node"], limit=50, parententry="$special:rootnode") %}

This also relates to #1488