SpringBoard: cell dragging and re-ordering renovation

[evadne]

Quoting https://github.com/AlanQuatermain/AQGridView/issues/129:

I’m seeing artifacts related to drop animation using the wrong rect (cell index off by one) in some edge cases in SpringBoard and zapping the cells prematurely, triggering a recreation process; those issues will be resolved later.

AQGridView needs a canonical cell dragging demo and the SpringBoard demo does not work properly after the ARC conversion.