MDSAL-298: properly handle unkeyed lists

MDSAL-298: properly handle unkeyed lists

Unkeyed lists are not representable in binding, which means they
cannot be reported as modified children, either. This has implications
for any data change which contains unrepresentable fields as child
modifications.

Previously we would report SUBTREE_MODIFIED for the container node,
but would fail to report any children, which is obviously wrong, as the
user is left guessing as to what exactly happened.

This patch modifies LazyDataObjectModification to report a WRITE event
if modifications to unrepresentable children are found in SUBTREE_MODIFIED
case. Since this is a potentially expensive operation, we cache a child
addressability summary in BindingCodecTreeNode, so that we go to this
slow path only when needed.

We also expose BindingStructuralType enumeration, so controller's
sal-binding-broker can reuse the implementation rather than having its
own copy.

Change-Id: I6642166cd262d0dddb1b2ed6d73a20785d0efff6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>