Fix possible stack overflow with many property bindings

Fix possible stack overflow with many property bindings

When there are a lot of bindings to the same property (like 20 000),
we would get stack overflows because the notify list for the
changed signal was traversed recursively.

Changing this also speeds up the traversal. I see something like
~40% reduction in the case of layout() for a notify list of around
200 items.

Note: To make it possible to traverse the double-linked list backwards,
the next-pointer needs to be moved to the beginning of the struct,
because the implementation pattern assumes this
(node->next->prev = &node->next).

I think this code has rotted after it was added, since the prev pointer
was never actually used anywhere before.

Change-Id: Icdfac50b7c8584a908efa65694c7f5f416cb153b
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>