[Qt]: fix num Blocks to maturity in Tx description
[Qt]: fix num Blocks to maturity in Tx description

- as we have main.h included in transactiondesc.cpp, we now also use
  COINBASE_MATURITY +  1 for the descriptive string
- fixes #3131

wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin