Make it obvious which cell is selected in main napari window

[dstansby]

From #60 (comment):

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: napari/napari#2560