Position new ui nodes correctly when existing dirty dead nodes exist

Position new ui nodes correctly when existing dirty dead nodes exist

If you add a new ui node that was recently removed it will still be
in the tree, but marked dirty. In this case we previously just used
the old node, which meant it wouldn't get the same position as if
the dirty nodes had been processed first (and deleted) before the
new node was added.

We handle this by detecting this case and reposition the node as if
it was new.

https://bugzilla.gnome.org/show_bug.cgi?id=603128