Make it possible to get the wallet log from Firefly

[Thoralf-M]

For debugging it would be very helpful to be able to get the wallet log also from a released Firefly version and not only during development.
Having a button in the settings to enable it would be nice. I think it should only be started then and the next time one starts Firefly one would need to click on it again if one wants it, because the log can very large if it's enabled all the time