iconGrid: Queue a relayout after child opacity changes

iconGrid: Queue a relayout after child opacity changes

We're using a vfunc override for `get_paint_volume` to exclude children
with an opacity of 0 from the paint volume and thus decrease the size of
the area we need to paint.

Now if the paint volume is requested during the spring animation (the
real icons are hidden using an opacity of 0 and clones are used for the
animation), `get_paint_volume` returns a paint volume with a height of
0. After that, the spring animation finishes and the icon-opacities are
set to 255 in `_resetAnimationActors`, and since we cache paint volumes
and there's no reason for Clutter to assume it got invalid, the icons
end up not being painted.

Fix this by queuing a relayout of the grid when the opacity of a child
is changed from or to 0, which manually invalidates the paint volume.

The reason why this is not an issue with the paginated icon grid
(all-apps view) is probably because StScrollView invalidates the paint
volume a lot more often than regular containers.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/1502