iconGrid/iconGrid: Wait for icon sizes before running spring animation

To run the spring animation, IconGrid uses the transformed position and size
of the icons. This works okay when IconGridLayout doesn't need to update the
icon sizes, but looks bad when IconGridLayout selects a different icon size.

Wait for the icon sizes to be recalculated, and the icons  beallocated, before
running the spring animation. If no icon size update is pending, run the spring
animation directly.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1518>

(cherry picked from commit b7308750)