UX improvements for Stax

[bigspider]

List of possible improvements for Stax UX.

• Display xpub:
  • It would be nice to have a smaller font for an xpub, and make sure it fits on a single screen.
• Register policy:
  • Remove the "Cosigner registered", registration only happens at the end; ideally, we could try to make it go to the next/previous cosigner just by going right/left (on all devices, not just for Stax), without having to confirm anything explicitly until the end.
  • Smaller font size (or adaptive based on the size of content) would be great.
  • Word wrapping for long policy names with spaces? Currently it splits at the end of the line, regardless if it makes sense to split there or not. (Lower priority, as this might be tricky)
• Sign_psbt:
  • Can fit everything on a single screen for simple transactions with just one output?
  • Going to the last screen shows a <-- button, but you can only go back to the previous step (showing the fees), which is awkward. Similarly to cosigners, we might want to allow going back and forth without confirming anything explicitly.

[bigspider]

The ones that were feasible were all done in #270.