This adds:
- a dialog after Create Unsigned, which lets you save a PSBT file in binary format, e.g. to an SD card
- a "Load PSBT" menu entry lets you pick a PSBT file. We broadcast the transaction if complete
Save flow
<img width="482" alt="Schermafbeelding 2020-01-04 om 20 39 34" src="https://user-images.githubusercontent.com/10217/71765684-ba60d580-2f32-11ea-8dea-0c4398eb6e15.png">
<img width="287" alt="Schermafbeelding 2020-01-04 om 20 40 35" src="https://user-images.githubusercontent.com/10217/71765677-a0bf8e00-2f32-11ea-8172-12dfd34a89f3.png">
<img width="594" alt="Schermafbeelding 2020-01-04 om 20 41 12" src="https://user-images.githubusercontent.com/10217/71765681-aa48f600-2f32-11ea-8e2c-c4f6bf9f5309.png">
<img width="632" alt="Schermafbeelding 2020-01-04 om 20 41 28" src="https://user-images.githubusercontent.com/10217/71765691-d19fc300-2f32-11ea-97ff-70f5dd59987a.png">
By default the file name contains the destination address(es) and amount(s).
We only use the binary format for files, in order to avoid compatibility hell. If we do want to add base64 file format support, we should use a different extension for that (.psbt64?).
Load flow
Select a file: <img width="649" alt="Schermafbeelding 2020-01-04 om 21 08 57" src="https://user-images.githubusercontent.com/10217/71766089-2ba28780-2f37-11ea-875d-074794b5707d.png">
Offer to send if complete:
<img width="308" alt="Schermafbeelding 2020-01-04 om 21 09 06" src="https://user-images.githubusercontent.com/10217/71766088-2a715a80-2f37-11ea-807d-394c8b840c59.png">
Tell user if signatures are missing, offer to copy to clipboard: <img width="308" alt="Schermafbeelding 2020-01-04 om 21 15 57" src="https://user-images.githubusercontent.com/10217/71766115-702e2300-2f37-11ea-9f62-a6ede499c0fa.png">
Incomplete for another reason:
<img width="309" alt="Schermafbeelding 2020-01-04 om 21 07 51" src="https://user-images.githubusercontent.com/10217/71766090-2c3b1e00-2f37-11ea-8a22-6188377b67a1.png">