iconGrid: Only animate items when we actually need it

iconGrid: Only animate items when we actually need it

Animating items of the iconGrid involves calling a few more C functions,
which is quite slow. Especially calling ClutterActor.set_easing_delay()
is slow because we override that function in JS to adjust for the
animation slow-down factor. So add a small class variable to make sure
we only animate the icons of the grid when we actually need it.

This makes the average time spent in vfunc_allocate() of the iconGrid go
down to about 0.7 ms.

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