More robust recursion in Group.getAllChildren()

[BenjaminDRichards]

The current implementation is hard-locked to just check for Group, via objType()==="Group". This means it doesn't play well with extreme group derivatives, such as the forthcoming buffered rendering plugin. It would be better to check for children that implement getAllChildren() themselves.

[BenjaminDRichards]

Addendum thought: There's a possibility of getting infinite loops if the group derivative doesn't prevent such behavior. For example, buffers can form simple feedback loops, although they cannot self-feedback in a single step. We should at least point out that getAllChildren() implementations should self-avert in some fashion.