Too many bitcoins allowed in amount. (#2401)

Using magic number doesn't seem right. Could we factor this out, together with https://github.com/bitcoin/bitcoin/blob/master/src/bitcoinrpc.cpp#L96 ?
And what about BitcoinUnits::parse() as well ?

src/qt/bitcoinamountfield.cpp

@@ -60,7 +60,9 @@ bool BitcoinAmountField::validate()
     bool valid = true;
     if (amount->value() == 0.0)
         valid = false;
-    if (valid && !BitcoinUnits::parse(currentUnit, text(), 0))
+    else if (!BitcoinUnits::parse(currentUnit, text(), 0))
+        valid = false;
+    else if (amount->value() > BitcoinUnits::maxAmount(currentUnit))
         valid = false;
 
     setValid(valid);

src/qt/bitcoinunits.cpp

@@ -63,6 +63,17 @@ qint64 BitcoinUnits::factor(int unit)
     }
 }
 
+qint64 BitcoinUnits::maxAmount(int unit)
+{
+    switch(unit)
+    {
+    case BTC:  return Q_INT64_C(21000000);
+    case mBTC: return Q_INT64_C(21000000000);
+    case uBTC: return Q_INT64_C(21000000000000);
+    default:   return 0;
+    }
+}
+
 int BitcoinUnits::amountDigits(int unit)
 {
     switch(unit)

src/qt/bitcoinunits.h

@@ -38,6 +38,8 @@ public:
     static QString description(int unit);
     //! Number of Satoshis (1e-8) per unit
     static qint64 factor(int unit);
+    //! Max amount per unit
+    static qint64 maxAmount(int unit);
     //! Number of amount digits (to represent max number of coins)
     static int amountDigits(int unit);
     //! Number of decimals left