Make #groups script more flexible

[marceltaeumel]

The current #groups script inserts text objects into a flat list. This affects re-use as shown below.

Maybe we could add a new object property to tell the script interpreter to ignore certain model nodes for child computation?

script := {
[:morphOrText |
   #text -> morphOrText asText.
   #skip -> morphOrText isText. "For script interpreter only."].
} asScript.