Displeasing gap between selected cells

[alexdima]

Testing #119118

When selecting multiple cells, there is an esthetically displeasing "gap" between adjacent selected cells:

This makes it difficult to immediately say if the selection is contiguous or there is an item not selected:

[rebornix]

Starting from @misolori

[miguelsolorio]

Yea this is because of the spacing between the cells and the bottom "add new cell" toolbar. Will see what we can do ti improve this. cc @jilljac