Optimize getAllChildrenIterable() performance. (#13803)

getAllChildrenIterable() is invoked frequently, its used by
attach/detach and markAsDirty.
Because of the implementation detail in CombinedIterator, on every call
all iterators are checked. This means that on every component which has
multiple childs, each next() call will first check the extension
iterator, before advancing in the child iterator.
By having the childs first, this overhead is reduced. If no extensions
are existing (which is quite common) the iterator is not added at all.
(creating an iterator on an empty UnmodifiableCollection is waste of
time and memory)

Change-Id: I23bb91464052ad4282963ec4b5be8a52c6847d4f