Subtract fee from the change output instead of the first output

[cygnet3]

Right now when sending a transaction, we subtract the fee from the very first transaction output.
We should instead remove it from the change output, so we don't confuse users that want to send a specific amount of money to a certain address.

When making this change we should take into account that the change output either might not exist, or may not cover the full fee amount.
In the former case, we should subtract it from the first output after all.
In the latter case, we should probably throw an error. Maybe we should increase the dust threshold to make sure this error occurs rarely.