Explicitly update widget a11y bounds when allocating

Explicitly update widget a11y bounds when allocating

The a11y machinery is using signal subscription to get notified of size
changes and notify listeners in turn. This is suboptimal for a couple of
reasons:

 - if something connects to the GtkWidget::size-allocate signal we need
   to emit it; currently, we have an optimization in place that will
   skip the signal emission if there are no handlers, and it would be
   nice to go through the fast path
 - the accessibility implementation is part of GTK, and should not go
   through additional hoops like any out-of-tree API consumer