Refactor screen scroll even more

[rlamorea]

Issue #45 cut screen scrolling speed to a third of the original algorithm, and is probably ok for now.

But there is a faster way.

Effectively delete or insert cells and let the rest of them remain untouched.
So if we scroll up one line:

• Delete the first cells from the cell array
• Insert cells at the end of the cell array

Scrolling left one column would, for each line:

• Insert one cell at 0 and every thereafter to end of screen
• Delete one cell at 1+ (adjusted for the insert) and every thereafter to end of screen

This would mean:

• removing the cell data-row/data-column attributes
• selecting cells by nth child (or keep cells in array and use array index)
• identifying selected cell by Array.from(parent.children).indexOf(child) approach