[Qt] Coin control cleanup because of non-rounding fees

#3959 introduced that fees are not rounded anymore up to 1000 bytes.
This results in that coin control guesses the fee wrong very often.
So I would like to make the change:
- add "~" in front of fee, afterfee and change; show a tooltip "Can vary +/- %1 satoshi(s) per input."

This line gives a compiler error since nFee is not used.