JoinMarketQt: Use restartForScan callback in selectWallet

[kristapsk]

Fixes #341.

[AdamISZ]

Many thanks for this work. Just to note what I understand here:

I believe this is the correct solution to the issue described in #341 , in that when a wallet is loaded for the first time on a Core instance (e.g. taken from another machine), it requires a rescan because those addresses are not imported here. However, unlike the case of a newly generated wallet (see the JMMainWindow.generateWallet function in joinmarket-qt.py), we didn't previously use the restartForScan callback as the value of restart_cb passed into the wallet sync process, meaning that when the function add_watchonly_addresses was being called in blockchaininterface.py it was being called with a null restart_cb, which in that function is interpreted basically as meaning this is a CLI application and so we can safely print a message and quit. But since it's not a CLI application that's not the right behaviour, and so @kristapsk 's solution here is the correct one: call loadWalletFromBlockchain with restart_cb set to self.restartForScan, exactly as for the newly generated wallet case.

[AdamISZ]

For context, it's important to note that the information message (same whether on CLI or Qt) given to the user is:

"restart Bitcoin Core with -rescan if you're "
"recovering an existing wallet from backup seed\n"
"Otherwise just restart this joinmarket application."

so in the case we're addressing here, a rescan on Core is actually needed whereas in the new wallet generation case it's not.

[AdamISZ]

Tested OK with a slight kludge: I set a different rpc_wallet_file to the one in which my test wallet was created, and successfully got the info message and the shutdown as expected. tACK.