state: Consider full balance transfer

[renaynay]

    [non-blocking]

Do we want a full balance transfer option? Or have we discussed how someone would do that?

I assume the full cost of this method is amount + gasFee. So it's hard for users to completely empty a wallet.
But if we can structure it to allow for amount being the full cost, and what is sent to the new address is amount - gasFee, then users can drain a wallet easily.

Something a simple as a subGass bool argument or just a second method.

Originally posted by @MSevey in #506 (comment)