Move value property to PCB_FOOTPRINT_FIELD_PROPERTY.

(Also reworks things a little to give the fields a
more expected order.)

include/properties/property_mgr.h

@@ -140,6 +140,14 @@ public:
      */
     PROPERTY_BASE& AddProperty( PROPERTY_BASE* aProperty, const wxString& aGroup = wxEmptyString );
 
+    /**
+     * Replace an existing property.  This is used to modify the implementation of a property without
+     * moving it in the display order.
+     * @param aProperty is the property to register.
+     * @param aGroup is an optional grouping key for the property
+     */
+    PROPERTY_BASE& ReplaceProperty( PROPERTY_BASE* aProperty, const wxString& aGroup );
+
     /**
      * Replace an existing property for a specific type.
      *