Optimize allocate() vfunc of the icon grid

Spent a bit of time playing around with the gjs profiler a bit and hacking together sysprof marks for the shell (MR incoming). Turns out that while the trampolining between C and JS is slow, it's not the only thing slowing us down, and we can also do a lot better caching things instead of recalculating them on every frame.

Start out by optimizing vfunc_allocate() of the iconGrid, getting the average time spent in that function on my machine down from 2.3 ms to ... drum roll ... 0.7 ms.