Cannot apply custom drag strategy to shadow in v11

[BeksOmega]

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Description

Custom drag strategies on shadow blocks do not work in v11 because we select the parent instead of the shadow.

This used to work, but was broken by #8138 because it was trying to fix #8137

We need a solution that allows shadow blocks to be the thing being dragged, but doesn't have it be the focus of dispose. I think we can just dispose of the root block instead, but I need to dig into it.

Additional info

This should go into our first v11 patch, since it's blocking for Lego and MakeCode.

cc @riknoll