Gui saves custom client id and loglevel

- gui NewWindow takes SimpleClientIdentity as argument
- gui NewWindow takes ethutil.ConfigManager as argument to manage flag persistence
- gui now saves loglevel and custom client id via config.Save
- rename custom client id methods consistently also in wallet.qml
- clientIdentifier now set in main wrappers
- version handled within wrapper
- modify InitConfig now returning *ethutil.ConfigManager (passed to gui)