Undo / redo support for mobile

[mdirolf]

This is a followup to #486 which just got merged.

The idea is to add an undo item to the "more" menu (and a redo, if the current state makes that a possibility). Hitting more > undo multiple times to go through history would probably be clunky. I propose that instead when one of those buttons gets hit we go into "history mode" which removes the autofill/clues/publish buttons from the top bar and replaces them with "undo", "redo", and "done". In addition any non-history action (like typing in a cell) would exit from history mode.

Edit: As per @legnes's suggestion we should put buttons on the soft keyboard's more page and disable them when they aren't relevant (based on history state).

[legnes]

That could be cool! I'm a little worried that replacing the top bar buttons might be confusing -- it may be more intuitive in its own "area"/span, but I'm not sure! Another possibility, could it live in the mobile keyboard More page?

[mdirolf]

Keyboard "more" page is 100% the correct way to go - great idea!