**Problem:**

Grid elements with `position: absolute` are always moved relative to the
cell they appear in.

**Fix:**

For cell elements that don't have explicit grid positioning (`gridRow` /
`gridColumn`), move them relative to the grid itself instead, and don't
show the cell target outline during the move.

(Elements that have positioning will still be absolutely positioned
relatively to their cell, and the positioning will be kept).

Fixes #6404