qt: Network-specific example address

Generate an (invalid) example address for in the bitcoin address
widgets, based on the network prefix, instead of hardcoding a mainnet
address.

• 1NS17iag9jJgTHD1VXjvLCEnZuQ3rJDE9L for mainnet (same as now)
• n2wxQmfexkjwEPgdD6iJA7T7RtzkmHxhFc for testnet