iconGrid: Get max child size using existing array of visible children

iconGrid: Get max child size using existing array of visible children

Using a preexisting array to iterate over is much faster than iterating
over the actors children using a "for ... of" loop, that's because the
latter calls into C functions to get the next actor all the time.

Also, stop using array deconstruction here, it turned out that this is
extremely expensive. When profiling this part of the code, it turned out
that only 9% of the time spent in _getChildrenMaxSize() is spent calling
the get_preferred_height/width() methods. When not using array
deconstruction, this time increased to 22%, still not great, but a lot
better.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1713>