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/bitcoinamountfield.h

@@ -8,17 +8,18 @@
 #include <QWidget>
 
 QT_BEGIN_NAMESPACE
-class QDoubleSpinBox;
 class QValueComboBox;
 QT_END_NAMESPACE
 
+class AmountSpinBox;
+
 /** Widget for entering bitcoin amounts.
   */
 class BitcoinAmountField: public QWidget
 {
     Q_OBJECT
 
-    Q_PROPERTY(qint64 value READ value WRITE setValue NOTIFY textChanged USER true)
+    Q_PROPERTY(qint64 value READ value WRITE setValue NOTIFY valueChanged USER true)
 
 public:
     explicit BitcoinAmountField(QWidget *parent = 0);
@@ -49,20 +50,15 @@ public:
     QWidget *setupTabChain(QWidget *prev);
 
 signals:
-    void textChanged();
+    void valueChanged();
 
 protected:
     /** Intercept focus-in event and ',' key presses */
     bool eventFilter(QObject *object, QEvent *event);
 
 private:
-    QDoubleSpinBox *amount;
+    AmountSpinBox *amount;
     QValueComboBox *unit;
-    int currentUnit;
-    qint64 nSingleStep;
-
-    void setText(const QString &text);
-    QString text() const;
 
 private slots:
     void unitChanged(int idx);