qt: show all units in send confirmation

Make users accustomed to the other subdivision units (mBTC, muBTC)
by showing the total amount in all units in the confirmation dialog.

This was recently raised on the mailing list and could be a preparation
for switching over the default unit eventually.
Merge pull request #3273

fb0507f qt: show all units in send confirmation (Wladimir J. van der Laan)