[Qt] RPC-Console: support nested commands and simple value queries

Commands can be executed with bracket syntax, example: getwalletinfo().
Commands can be nested, example: sendtoaddress(getnewaddress(), 10).
Simple queries are possible: listunspent()[0][txid]
Object values are accessed with a non-quoted string, example: [txid].

Fully backward compatible.
generate 101 is identical to generate(101)
Result value queries indicated with [] require the new brackets syntax.
Comma as argument separator is now also possible: sendtoaddress,<address>,<amount>
Space as argument separator works also with the bracket syntax, example: `sendtoaddress(getnewaddress() 10)

No dept limitation, complex commands are possible:
decoderawtransaction(getrawtransaction(getblock(getbestblockhash())[tx][0]))[vout][0][value]