iconGrid: Account for non-icon content and padding/spacing when scaling

_updateIconSizes() was assuming that the icon is the only content of
an item when scaling the icon size to ensure the item size matches
_fixedHItemSize/_fixedVItemSize. However the icon may have padding and
there might be a label and spacing between the icon and the label. This
resulted in items being larger than their slots.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2234