Make it obvious which cell is selected in main napari window

[dstansby]

From https://github.com/lowe-lab-ucl/arboretum/pull/60#issuecomment-1134838731:

If we could draw a white box (or something like this) around the cell in the middle panel so we can monitor which cell we initially clicked on & have highlighted on the tree, that'd be gold. Just thinking about ideas how to not lose track (pun intended) of which cell we're visualising.

[quantumjot]

Related issue: https://github.com/napari/napari/issues/2560