Wallet output descriptor load from and save to SD card

[tadeubas]

• Wallet output descriptor load from and save to SD card
• Fix screensaver button

[jdlcdl]

As a proof of concept, this worked for me, both saving and loading.

I realize that there are discussions of changing some ui with menus during the qr page. This is good, I think there is room for optimization in the ui, both while loading a descriptor and when saving; I look forward to participating in testing and discussions.

I had not realized that I can also save the xpub and zpub versions of a bip32 extended public key, but that works too.

[odudex]

Created a discussion about UI enhancements #280