Python: Make the All proxy traverse SimObject children as well

This patch changes the behaviour of the All proxy parameter to not
only consider the direct children, but also do a pre-order depth-first
traversal of the object tree and append all results from the
children.

This is used in a later patch to find all the memories in the system,
independent of where they are located in the hierarchy.

util/regress all passing (disregarding t1000 and eio)