Issue 5258 - Implement Filter::As<>() method to make working with filters easier from C++ code

compiled/subscription/UserDefinedSubscription.h

Index: compiled/subscription/UserDefinedSubscription.h
===================================================================
--- a/compiled/subscription/UserDefinedSubscription.h
+++ b/compiled/subscription/UserDefinedSubscription.h
@@ -22,28 +22,20 @@
 #include "../bindings/runtime.h"
 
 class UserDefinedSubscription : public Subscription
 {
 private:
   int mDefaults;
 
 public:
+  static constexpr Type classType = Type::USERDEFINED;
   explicit UserDefinedSubscription(const String& id);
   bool BINDINGS_EXPORTED IsDefaultFor(const Filter& filter) const;
   void BINDINGS_EXPORTED MakeDefaultFor(const Filter& filter);
   bool BINDINGS_EXPORTED IsGeneric() const
   {
     return mDefaults == 0;
   }
   void BINDINGS_EXPORTED InsertFilterAt(Filter& filter, unsigned pos);
   bool BINDINGS_EXPORTED RemoveFilterAt(unsigned pos);
   OwnedString BINDINGS_EXPORTED Serialize() const;
 };
-
-template<>
-inline UserDefinedSubscription* Subscription::As<UserDefinedSubscription>()
-{
-  if (mType != Type::USERDEFINED)
-    return nullptr;
-
-  return static_cast<UserDefinedSubscription*>(this);
-}