Restore bitcoin: URIs determining network address

Restored functionality where the network a Bitcoin URI address applies to is used to determine the network the client uses, altered such that invalid addresses do not incorrectly send the user to the test network. This avoids the previous issue where an invalid URI caused the client to open the wrong network, while also ensuring that the wallet opens a network appropriate to the URI where possible.

handleUriOrFile is the wrong place to be changing the network parameters. After all it can be called when the application is already running, and switching the parameters while the application is running could result in serious bugs.