UIUX: Improve entering recovery phrase screen

Two things would massively improve the UX here:
- Having a "Previous word" button besides the "Next word" button, jumping to the previous word. Few users will find out that you can actually change a word again by the `<` sign - and this would also erase a lot of entered words when going back from, say, word 10 to word 2 to change it. The alternative here would be to enable to jump to a previous word by the touch display, but I suppose this is much more difficult to implement @stepansnigirev?
- A cursor would help to orientate much better where one is currently entering what.