Plus42 algebraic expressions update

[Thomas Okken]

One difference with the 17B/27S that I should explain because it isn't obvious: in the list view, the rightmost two menu slots are unused on the original calculators, and I added white up-arrow and down-arrow keys there.

The new arrow keys move the currently displayed equations up or down in the list, swapping it with the one above or below. They will first show the equation currently at that spot, that is, the equation that the current one is going to be swapped with, and then they perform the swap and move the view position to that new location.

I don't think you can rearrange the order of the equations in the list on the original calculators, and that seemed like an annoying omission.