Allow better subclassing of MoneyField

Move generation of NumberField from constructor to method to allow override in subclass.

src/Forms/MoneyField.php

@@ -59,11 +59,7 @@ public function getAmountField()
     public function __construct($name, $title = null, $value = "")
     {
         $this->setName($name);
-        $this->fieldAmount = NumericField::create(
-            "{$name}[Amount]",
-            _t('SilverStripe\\Forms\\MoneyField.FIELDLABELAMOUNT', 'Amount')
-        )
-            ->setScale(2);
+        $this->buildAmountField();
         $this->buildCurrencyField();
 
         parent::__construct($name, $title, $value);
@@ -75,6 +71,18 @@ public function __clone()
         $this->fieldCurrency = clone $this->fieldCurrency;
     }
 
+    /**
+     * @return void
+     */
+    protected function buildAmountField()
+    {
+        $this->fieldAmount = NumericField::create(
+            "{$this->name}[Amount]",
+            _t('SilverStripe\\Forms\\MoneyField.FIELDLABELAMOUNT', 'Amount')
+        )
+            ->setScale(2);
+    }
+
     /**
      * Builds a new currency field based on the allowed currencies configured
      *