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/qt/bitcoinunits.h

@@ -120,6 +120,9 @@ public:
         return text;
     }
 
+    //! Return maximum number of base units (Satoshis)
+    static qint64 maxMoney();
+
 private:
     QList<BitcoinUnits::Unit> unitlist;
 };