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

compiled/filter/Filter.cpp

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 104 matching lines @@
   auto knownFilter = knownFilters.find(text);
   if (knownFilter)
   {
     knownFilter->second->AddRef();
     return knownFilter->second;
   }
 
   FilterPtr filter;
   switch (type)
   {
-    case Filter::Type::COMMENT:
+    case CommentFilter::classType:
       filter = FilterPtr(new CommentFilter(text), false);
       break;
-    case Filter::Type::INVALID:
+    case InvalidFilter::classType:
       filter = FilterPtr(new InvalidFilter(text, error), false);
       break;
-    case Filter::Type::BLOCKING:
+    case BlockingFilter::classType:
       filter = FilterPtr(new BlockingFilter(text, data.regexp), false);
       break;
-    case Filter::Type::WHITELIST:
+    case WhitelistFilter::classType:
       filter = FilterPtr(new WhitelistFilter(text, data.regexp), false);
       break;
-    case Filter::Type::ELEMHIDE:
+    case ElemHideFilter::classType:
       filter = FilterPtr(new ElemHideFilter(text, data.elemhide), false);
       break;
-    case Filter::Type::ELEMHIDEEXCEPTION:
+    case ElemHideException::classType:
       filter = FilterPtr(new ElemHideException(text, data.elemhide), false);
       break;
-    case Filter::Type::ELEMHIDEEMULATION:
+    case ElemHideEmulationFilter::classType:
       filter = FilterPtr(new ElemHideEmulationFilter(text, data.elemhide), false);
       if (static_cast<ElemHideEmulationFilter*>(filter.get())->IsGeneric())
         filter = FilterPtr(new InvalidFilter(text, u"filter_elemhideemulation_nodomain"_str), false);
       break;
     default:
       // This should never happen but just in case
       return nullptr;
   }
 
   // This is a hack: we looked up the entry using text but create it using
   // filter->mText. This works because both are equal at this point. However,
   // text refers to a temporary buffer which will go away.
   enter_context("Adding to known filters");
   knownFilter.assign(filter->mText, filter.get());
   exit_context();
 
   return filter.release();
 }