qt: Use fixed-point arithmetic in amount spinbox

Fixes various issues and cleans up code

- Fixes issue #4500: Amount widget +/- has floating point rounding artifacts
- Amount box can now be emptied again, without clearing to 0

Also aligns the amount to the right, as in other places.

src/Makefile.qt.include

@@ -145,6 +145,7 @@ BITCOIN_MM = \
 
 QT_MOC = \
   qt/bitcoin.moc \
+  qt/bitcoinamountfield.moc \
   qt/intro.moc \
   qt/overviewpage.moc \
   qt/rpcconsole.moc