[Qt] Improve single step in bitcoinamountfield

BEFORE:
Single step in sendcoins/receive/options dialog:
BTC: 100000 satoshis
mBTC: 100 satoshi
uBTC: 1 satoshi

AFTER:
Single step in sendcoins/receive dialog:
BTC/mBTC/uBTC: 100000 satoshis

Single step in options dialog:
BTC/mBTC/uBTC: CTransaction::nMinTxFee