Import-export PSBT transactions

[stepansnigirev]

This PR makes possible cooperative multisig signing.

Now if multiple people are holding the keys and using their own specters they can share partially signed transactions with each other.

Changes:

• adds a new "Import" tab in send menu where user can enter PSBT transaction received from someone else
  • parses and automatically detects the destination address and amount
  • checks what device already signed this transaction based on partial signatures present in the psbt
  • saves psbt transaction as pending
• updates raw psbt textarea field in the transaction details when a new signature is acquired
• checks the checkbox of the device that added the last signature
• fixes "delete transaction" action from the pending transaction screen
• detects signed devices from psbt itself, so correctly calculates signed devices and number of signatures now

The flow then is:

• Creator makes a transaction, optionally signs it and sends to all participants of the multisig setup
• Everybody signs and sends back signed transactions
• Creator or anyone else can use "Paste signed PSBT" button to finalize transaction

[stepansnigirev]

Added import of the external PSBTs, updated the description above.

[ben-kaufman]

Tested it a bit, works great, just needs to make sure if an error arises there should be flashed an error message instead of the error screen (I marked this in comment).

[ben-kaufman]

Should add try/ except to catch errors...

[stepansnigirev]

Thanks, updated!