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.cpp

@@ -4,6 +4,8 @@
 
 #include "bitcoinunits.h"
 
+#include "core.h"
+
 #include <QStringList>
 
 BitcoinUnits::BitcoinUnits(QObject *parent):
@@ -250,3 +252,8 @@ QVariant BitcoinUnits::data(const QModelIndex &index, int role) const
     }
     return QVariant();
 }
+
+qint64 BitcoinUnits::maxMoney()
+{
+    return MAX_MONEY;
+}