iconGrid: Cache max size of children

iconGrid: Cache max size of children

We call this._getChildrenMaxSize() from the allocate() vfunc of
IconGridLayout. Since the function is quite expensive, it slows the
layout process down a lot, so instead of re-calculating it on every
relayout, cache the max size of children.

This makes the average time spent in vfunc_allocate() of the iconGrid go
down from 2.3 ms to 1.6 ms.

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