iconGrid: Save class variables to local ones

iconGrid: Save class variables to local ones

It's quite slow to access class variables in JS, especially when they're
backed by GObject properties. To avoid accessing them in every iteration
when we're looping through the children of iconGrid, store those values
to another variable and reuse that inside the loop.

This shaves off another 0.2 ms from iconGrids vfunc_allocate(), getting
the average time spent in that function down from 1.3 ms to 1.1 ms.

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