Batch sign and broadcast chained transactions

[arobsn]

Signing details

1. ergo.sign_tx should take an array of UnsignedTransaction;
2. Show only one popup with all transaction details and xxx requests to sign n transactions;
3. ergo.sign_tx should return an array of SignedTransaction in the same order as taken in step 1;
4. Consider limiting the number of transactions to prevent abuse.

Broadcasting details

1. ergo.submit_tx should take an array of SignedTransaction;
2. Service worker should sort and send the transactions;
3. ergo.submit_tx should return an array of TransactionId in the same order as taken in step 1.