Description
A simple container element. Each group will
start its own code generation buffer and
inject itself into the generated items.
It must propagate down to its children.
Only the SVG element creates a local buffer;
group shares the buffer with any other element.
The propagation is done in two steps: the group
element propagates itself through the property
group. A child-group overwrites this value with
its own group and uses the received group as a
parent property.