Rework touch drag operation to render dragging tile as a React component

Rework touch drag operation to render dragging tile as a React component

We encountered a bug where a touch drag operation was interrupted, by
some operation, leaving a ghost tile left around half-way through the
drag operation. One option to fix this would be to implement a
tounchCancel handler to clean things up.

But an alternate approach, in this commit, is to render the tile being
dragged as a React component. This way, everything is rendered
uniformly as React components, and we have less dependence on precise
browser semantics with respect to touchCanel, etc.