box-layout: Remove the allocations hash table

box-layout: Remove the allocations hash table

The BoxLayout uses a HashTable to map the latest stable allocation of
each child, in order to use that as the initial value during an
animation; this in spite of already having a perfectly valid per-child
storage as part of the layout manager: ClutterBoxChild.

The last stable allocation should be stored inside the ClutterBoxChild
instead of having it in the private data for ClutterBoxLayout. The
access remains O(1), since there is a 1:1 mapping between child and
BoxChild instances, but we save a little bit of memory and we avoid
keeping aroud allocations for old children.